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>’;
}
});