By default, a form compliance policy is already present in each campaign's properties which only needs to be enabled if you need to.

First, go to your Campaigns list. Then find the campaign that you want to enable the compliance policy. Open up its Properties.

Then go to See More > GDPR Compliance tab. 

Then you'll see the section where you can enable the compliance details on the form as well as edit it. See screenshot below:

This is what it would look on the form:

If you've set the compliance as required, this prompt will pop up when customers forget to tick the agreement check box: