I assume you are referring to Microsoft Forms. If so, are you looking for the ID of forms they created, or the ID of forms they submitted?
If you are looking for the ID of forms they created, you will need to have access to the user's account so you can open MS Forms and open the form they created. When you open the form, the ID will be in the URL after &id=.
If you are looking for the ID of their submissions, open the form and click on Responses in the menu. Then choose Open in Excel to the right of the View results button. When it exports to Excel it will list the ID of each submission along with their e-mail address and name. I checked several of my forms and entries from people who are no longer with the company, and they are still listed.