How to add Dropdown menu in Subject Field in Pirate Forms

Add the following code snippet in your functions.php:

add_filter( 'pirate_forms_public_controls', 'pf_add_subject_dropdown' );
function pf_add_subject_dropdown( $elements ) {
	foreach ( $elements as $key => $element ) {
		if ( $element['id'] === 'pirate-forms-contact-subject' ) {
			$elements[ $key ]['type']    = 'select';
			$elements[ $key ]['options'] = array(
				'My subject option 1' => 'My subject option 1',
				'My subject option 2' => 'My subject option 2',
				'My subject option 3' => 'My subject option 3'
			);
		}
	}

	return $elements; 
}

You need to change options with your desired options.