Please attach the file.
You should be able to use the same onclick event to set the value of the hidden form element:
onclick="document.getElementById('WADADeleteRecordID').value=<?php echo($WADAlaromedel->getColumnVal("id")); ?>;"
Then as long as the button has the same name and submits the form it should work. I think you can have both a data-target and onclick on the same button. The data-target is for opening the model, so that part is removed from the onclick.