Hi Milos,
On the room page, it takes the English name 'book-now' to show that button.
You can edit file: /templates/gk_royal/html/com_content/article/room.php
Look for this code:
if ($field->name === 'book-now') {
$booknow = $field->rawvalue;
$booknowlabel = $field->title;
}
and change the 'book-now' to the alias of your new field (I don't find it now in backend of your site)