I'm trying to create a variable external link or redirect, but I'm complete beginner with php...

1.In one instance I have a script for registration (address, desciption, features, etc) with Post--> I am trying to add another field for URL --> I have added this extra line and extra line in MySql database.... "property_link"

2. In another instance I have a script with Constructed link which currently point to "Booking Form" (depending on the item(property) --> but I need to replace with external link from registration Form for each item....
- But I don't understand How to replace/construct it?!
Now it looks like
//$url=SITEPAGE_URL."&task=dobooking&selectedPropert y=$property_uid}";// --> and everyone tells me I should create the new link with something like $_POST['property_link']; - but when I try, it doesn't work How it should be joined/redirected

how to combine:
$url=SITEPAGE_URL."&task=dobooking&selectedPropert y=$property_uid}";+ $_POST['property_link'];

Or - How to create URL with matching item page+ external url from registration data ?