I have tried to create a webhook in a custom action. The dialog to enter the name and the url shows up, but when I click in one of the fields the dialog closes.
The workaround is to create a button and create the webhook with that button. Then I could choose it in my custom action.
The web hook didn’t work when the app is running as installed app on my ipad mini.
On my Android Phone and on my windows machine is the webhook excecuted like expected.
Regarding the dialog disappearing, there is a “funny”. You can work around it by using TAB to move through the dialog fields and finally to the Save button.