To use the Application User System Roles Edit Form, add a Showdataform action to the application user page as seen the steps/screenshots below.

1) On the application user page go to design mode > Edit action groups 
2) Edit an action group or add a new one.
3) In the Action group form, go to Actions > Click the ellipses to go into the list of actions in the action group
4) Click add to add a new action
5) Build the action seen in the second screenshot below > Click OK.
6) Click on the newly added action and mark the checkboxes to add the desired roles to an application user.

Steps 1-3:
Steps 1-3

Steps 4-6:
Steps 4-6

Created form:
Using the new form to add security roles

Unfortunately, there aren’t checkboxes on the data form to add site and constituent security functions. There is a suggestion on the Idea Bank to add site and constituent security options to this data form