The reason you are not seeing the custom claim is that it looks like you created an Auth0 action in the Post Login trigger, but you are requesting the token using Client Credential flow.
If you need custom claims during the Client Credentials flow, you should create an action in the Credential Exchange trigger (for Machine to Machine\Client Credential flow).
If you need custom claims after login (i.e., for the Access token after login), you should use the Post Login trigger.