That field is not avaiable to be used on forms, in both Forms 1.0 and 2.
If you want to mark all leads who fill out that form to be Marketing Suspended, best to add them all to a Static List. Create a Smart Campaign to Trigger Added to Staitic list. In the flow, use Change Data Value and here you can select Marketing Suspended.
When all fill out that form they will be marked as suspended.