I found the solution.
I am writing the solution here so that if anyone needs it, it can be used:
add_filter( ‘fep_form_fields’, function( $fields ){
	$fields[‘cus_fep_textbox41’] = [
		‘type’		 => ‘select’,
		‘where’    => ‘newmessage’,
		‘priority’	 => 12,
		‘label’		 => ‘Title’,
		‘options’	 => array(
					‘test1’	=> __( ‘Test1’),
					‘test2’=> __( ‘Test2’),
				),
	];
	return $fields;
});
add_action( ‘fep_action_message_after_send’, function( $message_id, $message, $new_message ){
	if ( ! empty( $message[‘cus_fep_textbox41’] ) ) {
		fep_add_meta( $message_id, ‘cus_fep_textbox41’, $message[‘cus_fep_textbox41’], true );
	}
}, 10, 3);
add_action( ‘fep_display_after_message’, function(){
	if ( $cus_fep_textbox41 = fep_get_meta( fep_get_the_id(), ‘cus_fep_textbox41’, true ) ) {
		echo ‘<div>Ville :      ‘; echo esc_html( $cus_fep_textbox41 ) . ‘</div>’;
	}
});