How do I create a test app with the addon callback URL set to localhost?

Hello,

I am trying to create a new integration that uses oauth and the self hosted web service function. This requires the oauth2 callback URL to be set.

However, trying to set it to https://localhost:3000/partners/servicem8/callback does not seem to work (this is only for testing) and it instead returns a 403 error - how can I set it to localhost so that I am able to test my callback?

Thanks.