This is for the administrators of Teams account.
If you do not see the Incoming Webhook items in the [Receiving Visitor Notifications in Microsoft Teams] section, you can display it in the Microsoft Teams admin center by following these steps below.
Approval for Incoming Webhook
Log in to the Microsoft Teams admin center as an administrator.
Click [Teams apps] > [Manage apps] from the side menu.
Search for "Incoming Webhook" in the search box.
Click "Incoming Webhook".
Set the status switch to "Allowed".
Approval for third-party apps
Click [Teams apps] > [Permission Policies] from the side menu.
Click your organization in the App permission policies.
Change the "Third party apps" field to "Allow all apps" and click "Save".
This is the end for the setting.