diff --git a/src/Checkout.js b/src/Checkout.js
index 77c198f..55e56c9 100644
--- a/src/Checkout.js
+++ b/src/Checkout.js
@@ -1,6 +1,27 @@
 import * as CONFIG from './config';
 import * as ENDPOINTS from './index';
 
+/**
+ * Determine the full URL based on the environment and subdomain.
+ * 
+ * @param {string} environment 
+ * @param {object} options 
+ * @returns {string}
+ */
+const determineUrl = (environment, options) => {
+    const apiUrl = new URL(environment);
+
+    if (options && options.subdomain) {
+        const subdomain = options.subdomain;
+        if (typeof subdomain === 'string' && /^[0-9a-z]{8,11}$/.test(subdomain)) {
+            const { protocol, port, hostname } = apiUrl;
+            return new URL(`${protocol}//${subdomain}.${hostname}${port ? `:${port}` : ''}`).toString().slice(0, -1);
+        }
+    }
+
+    return apiUrl.toString().slice(0, -1);
+};
+
 const determineHost = (key, options) => {
     // If specified, use the custom host
     if (options && options.host) {
@@ -17,9 +38,9 @@ const determineHost = (key, options) => {
             (process.env.CKO_ENVIRONMENT &&
                 process.env.CKO_ENVIRONMENT.toLowerCase().trim() === 'live')
         ) {
-            return CONFIG.LIVE_BASE_URL;
+            return determineUrl(CONFIG.LIVE_BASE_URL, options);
         }
-        return CONFIG.SANDBOX_BASE_URL;
+        return determineUrl(CONFIG.SANDBOX_BASE_URL, options);
     }
     // Priority 2: oAuth declared vars
     if (options && options.client) {
@@ -28,9 +49,9 @@ const determineHost = (key, options) => {
             (options.environment && options.environment.toLowerCase().trim() === 'production') ||
             (options.environment && options.environment.toLowerCase().trim() === 'live')
         ) {
-            return CONFIG.LIVE_BASE_URL;
+            return determineUrl(CONFIG.LIVE_BASE_URL, options);
         }
-        return CONFIG.SANDBOX_BASE_URL;
+        return determineUrl(CONFIG.SANDBOX_BASE_URL, options);
     }
 
     // Priority 3: MBC or NAS static keys
@@ -39,8 +60,8 @@ const determineHost = (key, options) => {
         key = key.replace('Bearer', '').trim();
     }
     return CONFIG.MBC_LIVE_SECRET_KEY_REGEX.test(key) || CONFIG.NAS_LIVE_SECRET_KEY_REGEX.test(key)
-        ? CONFIG.LIVE_BASE_URL
-        : CONFIG.SANDBOX_BASE_URL;
+        ? determineUrl(CONFIG.LIVE_BASE_URL, options)
+        : determineUrl(CONFIG.SANDBOX_BASE_URL, options);
 };
 
 const determineSecretKey = (key) => {
@@ -133,6 +154,7 @@ export default class Checkout {
             headers: options && options.headers ? options.headers : {},
             access: undefined,
             httpClient: options && options.httpClient ? options.httpClient : undefined,
+            subdomain: options && options.subdomain ? options.subdomain : undefined,
         };
 
         this.payments = new ENDPOINTS.Payments(this.config);
diff --git a/test/config/config.js b/test/config/config.js
index 5fd2c9b..6e7f86f 100644
--- a/test/config/config.js
+++ b/test/config/config.js
@@ -139,7 +139,52 @@ describe('NAS oAuth', () => {
         expect(cko.config.agent).to.be.undefined;
     });
 
-    it('should default to sandbox  and gateway scope with oAuth credentials', () => {
+    it('should initialize with oAuth credentials with subdomain', () => {
+        const cko = new Checkout('2p7YQ37fHiRr8O6lQAikl8enICesB1dvAJrpmE2nZfEOpxzE-', {
+            client: 'ack_vvzhoai466su3j3vbxb47ts5oe',
+            scope: ['gateway'],
+            environment: 'sandbox',
+            subdomain: '12345domain'
+        });
+        expect(cko).to.be.instanceOf(Checkout);
+        expect(cko.config.client).to.equal('ack_vvzhoai466su3j3vbxb47ts5oe');
+        expect(cko.config.host).to.equal('https://12345domain.api.sandbox.checkout.com');
+        expect(cko.config.scope[0]).to.equal('gateway');
+        expect(cko.config.secret).to.equal('2p7YQ37fHiRr8O6lQAikl8enICesB1dvAJrpmE2nZfEOpxzE-');
+        expect(cko.config.agent).to.be.undefined;
+    });
+
+    it('should initialize with oAuth credentials with bad subdomain', () => {
+        const cko = new Checkout('2p7YQ37fHiRr8O6lQAikl8enICesB1dvAJrpmE2nZfEOpxzE-', {
+            client: 'ack_vvzhoai466su3j3vbxb47ts5oe',
+            scope: ['gateway'],
+            environment: 'sandbox',
+            subdomain: '12345domain12345'
+        });
+        expect(cko).to.be.instanceOf(Checkout);
+        expect(cko.config.client).to.equal('ack_vvzhoai466su3j3vbxb47ts5oe');
+        expect(cko.config.host).to.equal('https://api.sandbox.checkout.com');
+        expect(cko.config.scope[0]).to.equal('gateway');
+        expect(cko.config.secret).to.equal('2p7YQ37fHiRr8O6lQAikl8enICesB1dvAJrpmE2nZfEOpxzE-');
+        expect(cko.config.agent).to.be.undefined;
+    });
+
+    it('should initialize with oAuth credentials with subdomain empty', () => {
+        const cko = new Checkout('2p7YQ37fHiRr8O6lQAikl8enICesB1dvAJrpmE2nZfEOpxzE-', {
+            client: 'ack_vvzhoai466su3j3vbxb47ts5oe',
+            scope: ['gateway'],
+            environment: 'sandbox',
+            subdomain: ''
+        });
+        expect(cko).to.be.instanceOf(Checkout);
+        expect(cko.config.client).to.equal('ack_vvzhoai466su3j3vbxb47ts5oe');
+        expect(cko.config.host).to.equal('https://api.sandbox.checkout.com');
+        expect(cko.config.scope[0]).to.equal('gateway');
+        expect(cko.config.secret).to.equal('2p7YQ37fHiRr8O6lQAikl8enICesB1dvAJrpmE2nZfEOpxzE-');
+        expect(cko.config.agent).to.be.undefined;
+    });
+
+    it('should default to sandbox and gateway scope with oAuth credentials', () => {
         const cko = new Checkout('2p7YQ37fHiRr8O6lQAikl8enICesB1dvAJrpmE2nZfEOpxzE-', {
             client: 'ack_vvzhoai466su3j3vbxb47ts5oe',
         });
@@ -148,7 +193,7 @@ describe('NAS oAuth', () => {
         expect(cko.config.scope).to.equal('gateway');
     });
 
-    it('should default to sandbox  and gateway scope with oAuth credentials from env vars', () => {
+    it('should default to sandbox and gateway scope with oAuth credentials from env vars', () => {
         process.env.CKO_SECRET = '2p7YQ37fHiRr8O6lQAikl8enICesB1dvAJrpmE2nZfEOpxzE-';
         process.env.CKO_CLIENT = 'ack_vvzhoai466su3j3vbxb47ts5oe';