For an External Integration, the current setup is to use the Addon Activation URL as the endpoint users hit when they click Connect, and that endpoint should start your OAuth flow.
The OAuth redirect_uri is the URL ServiceM8 sends the user back to after authorisation, and the same redirect_uri must also be used when you exchange the temporary code for an access token.
In the current developer UI, there may not be a separate field labelled Return URL, so use the Addon Activation URL field for the host ServiceM8 validates against.
For your example, set the Addon Activation URL to an HTTPS endpoint on the same host as your callback, such as https://rhijyaoguokspapkwtrt.supabase.co/..., then pass https://rhijyaoguokspapkwtrt.supabase.co/functions/v1/servicem8-callback as the OAuth redirect_uri.
The Callback URL for Web Service Hosted add-ons is separate from the OAuth return URL and is used for ServiceM8 to invoke your hosted add-on in response to actions or events.
If it still won’t validate after the host matches, email support@servicem8.com with the App ID so we can check the Store Connect item privately.