It will require a bit customization here:
1/ Edit file: ROOT/templates/ja_trip/etc/extrafields/tour-info.xml
and add this line of code:
<field name="booklink" type="text" label="TPL_BOOKLINK_LABEL" description="TPL_BOOKLINK_DESC"/>
2/ Add this code to the file: ROOT/templates/ja_trip/html/com_content/article/tour.php
$booklink = $extrafields->get('booklink');
.....
<?php if(!empty($booklink)) :?>
<div class="btn-actions mt-5">
<a class="btn btn-primary w-100" href="<?php echo $booklink;?>" title="Book Now">Book Now for <?php echo $price; ?></a>
</div>
<?php endif ;?>