From https://github.com/OfficeDev/Office-Add-in-samples/blob/main/Samples/hello-world/outlook-hello-world/README.md

Configure a localhost web server and run the sample from localhost

If you prefer to configure a web server and host the add-in's web files from your computer, use the following steps.

1. Install a recent version of npm and Node.js on your computer. To verify if you've already installed these tools, run the commands node -v and npm -v in your terminal.

2. You need http-server to run the local web server. If you haven't installed this yet you can do this with the following command:

   npm install --global http-server

3. You need Office-Addin-dev-certs to generate self-signed certificates to run the local web server. If you haven't installed this yet you can do this with the following command:

   npm install --global office-addin-dev-certs

4. Clone or download this sample to a folder on your computer. Then, go to that folder in a console or terminal window.

5. Run the following command to generate a self-signed certificate to use for the web server.

   npx office-addin-dev-certs install

   This command will display the folder location where it generated the certificate files.

6. Go to the folder location where the certificate files were generated. Copy the localhost.crt and localhost.key files to the cloned or downloaded sample folder.

7. Run the following command.

   http-server -S -C localhost.crt -K localhost.key --cors . -p 3000

   The http-server will run and host the current folder's files on localhost:3000.

8. Now that your localhost web server is running, you can sideload the manifest.xml file. Using the manifest.xml file, follow the steps in Sideload Outlook add-in on Windows or Mac to sideload and run the add-in.