To create a Google Chat notification webhook, make sure you're logged into your g-suite account instead of your personal e-mail. Personal e-mail addresses cannot create webhooks in their spaces.
Start by heading over to https://chat.google.com/ and make sure you're logged into your g-suite account. Make sure you have a space created, if you do, click that space and press the arrow down and press "Manage webhooks":
After creation, you'll get the URL by clicking the copy button:
