Was looking into how to add a custom button to the entity form, and found this thread in the community forums:
https://community.dynamics.com/crm/f/117/t/274955
So with a little modification(class attribute of the new button element), just so the second button looks similar to the default “submit” (which is re-labelled to “Subscribe” on the screenshot below), here is the script:
$(document).ready(function(){
$(‘#UpdateButton’).after(‘<button id=”deletebutton” type=”button” class=”btn btn-primary button” style=”margin-left:10px;”><span></span> Unsubscribe</button>’);
$(“#deletebutton”).click(function(){
$(‘#ita_subscribe_0’).attr(‘checked’, ‘checked’);
$(‘#UpdateButton’).click();
});
}
Then you can do whatever you need in that onclick javascript (set field values etc), and, eventually, just pass onclick to the default UpdateButton.