@Schultheis, Jakob (Admin) Thanks for reaching out. Please confirm if my understanding is correct. You have event grid subscription and webapps as the event grid webhook URL and when deploying the resource your event grid resource failed due to the handshake validation? Is my understanding correct?
If it is the above case, then your webapps resource should first be created and your event grid resource should only create once the webapp resource is successfully created so you need to leverage the dependsOn for this dependency.
In case if I misunderstood your ask then please share your workflow between different service/more details so I can assist you.