In DSM, there is a section called Pages. It is located in the left side menu under Settings. If you click Add New, You will see

Agreement – Creates a box when creating an account. This box will not allow the user to proceed with creating an account without ticking the box.

Student Agreement – Will not let the user proceed after adding student to the account without ticking the box.

Checkout Agreement – Will not let the user proceed checking out without ticking the box.

Update May 27, 2019:

An option has been added such that you can require your clients to sign a document online (with their mouse or finger), and that signed doc will be stored as a PDF that you can access in the future. you can see in the image below (Settings > Pages)