How to make the trigger button for the DP Modal popup full width?

You may use the below custom CSS in your WordPress Dashboard >> Divi >> Theme Options >> General >> Custom CSS area to make the trigger button full-width of the DP Modal module of the Divi Plus plugin:

a.et_pb_button.dipl_modal_trigger_element.dipl_modal_trigger_button {
width:100%!important;
text-align:center!important;
}