To start the process navigate to Toolbox - Form Manager and search for the form that needs to be edited, then select "Customize" or "Edit":

Once inside the form, select New Form Field, like this:

The new Form Field most include a Field Label Name, Database Variable Name, and the Field Type should be Checkbox Select: (to learn more about these fields types navigate to In-Depth Look Into Form Fields And Their Settings)

Once the Checkbox Select option has been chosen, the checkbox options need to be added to the following field:

Once the field has been added, the members of the directory will be able to choose the options added to this new form field once they are completing their profile: