Add webhook dialog: Add button inactive when user inputs webhook url

Description

Steps to reproduce:
1. Create a form.
2. Open the form in All Properties mode.
3. Switch to the Settings tab.
4. Open the Add Webhook dialog.
5. Type a webhook url.

Expected:
Add button is active.

Actual:
Add button is inactive, which confuses the user. The user must click outside the Url input field to make the button active.