Currently, users may not be able to create connectors in Microsoft Teams Desktop client. This is a known issue in Teams recently and engineering team is working on to fix this.
However, it should work in Teams web version. If you are clicking "done" button in this stage and seeing the white screen. Please try clicking the "home" button instead. Your Webhook would be created in either case.
We have issue with Done button, hence it won't work. Engineering team is working on fixing it. It does not affect creation of Connector.