Followed the Development Instructions for “write your own extension” and ran the wt create command to deploy my Sample Simple extension. Got ‘Cannot GET /extension’ error at webtasks url.
To my knowledge, the process to create your own custom extensions is no longer formally documented in the reference documentation as a custom extension would then not have any type of support and anything you can do with an extension you can also do with a regular client application registered in your Dashboard.
In conclusion, this is something unsupported and the actual question is also very generic and likely points to an issue with the custom code. My recommendation would be to treat this as a normal client application instead of an extension.