SharePoint Online provides the option of publishing a Microsoft Form for collecting responses and showing responses. After a form is created, it is shared to obtain a link to distribute to others or use with SharePoint Online. It may look something like
https://forms.microsoft.com/Pages/ResponsePage.aspx?id=<a_long_id>
When this base URL is used in the Microsoft Forms Web Part, the following error occurs:
This is not a valid form link. Please copy a URL from Microsoft Forms.
It looks something like the following:

To correct the error, change the hostname forms.microsoft.com to forms.office.com:
