Prepend Live Merge Tags with Dash Character

gppa-prepend-lmt-with-dash.php

<?php
// Update "123" with your form ID and "4" with your source field ID.
add_filter( 'gppa_live_merge_tag_value_123_4', function( $value, $merge_tag, $form, $field_id, $entry_values ) {
	if ( !$value ) {
		return $value;
	}
	return "-" . $value;
}, 10, 5 );