In a custom app with Oauth, during onAppInstall event I am trying to make a POST request call to a third party API with Basic Auth for which I have to use the Oauth Access Token as username and X as password.
You are facing this issue due to recent security updates in our platform, you can fix this issue by whitelisting the domain in the manifest.json file, you can follow the Whitelisted-Domains section in this link to learn how to whitelist your domain.
you can also check out more about our recent security updates in the below forum threads,
Sometimes Oauth apps might use a different redirect URL/domains to handle authentication services, could you please check in the networks tab if any other URL needs to be whitelisted?
also, if possible could you send the complete app for us to debug the issue? if you cannot share the app in the public, you can send it via the forum DM