This is a sample of how you can use the Valo Extensibility Framework @valo/extensibility.
Documentation about the framework can be found here: Modern Valo Extensibility Documentation.
Once the solution is deployed on your tenant, you can add it manually like this (uses the Office 365 CLI):
o365 spo customaction add -t "custom-valo-ui-extensions" -c 7d190fa2-c4ae-42b2-8537-431bf7bb8b41-u https://<your-site-URL> -l "ClientSideExtension.ApplicationCustomizer" -n "custom-valo-ui-extension"
m365 spo customaction add --name 'ValoCustomToolboxAction' --title "custom-valo-ui-extensions" --location "ClientSideExtension.ApplicationCustomizer" --clientSideComponentId 7d190fa2-c4ae-42b2-8537-431bf7bb8b41 --clientSideComponentProperties '{}' --url "https://simedev027.sharepoint.com/sites/tea-point-whats-new"