You can add it like this
header('Location: http://linkhere.com/'.$url_endpoint);
Try using double quotes and keeping the L in location lowercase...
L
location
header("location: http://linkhere.com/HERE_I_WANT_THE_VARIABLE");
or for example
header("location: http://linkhere.com/$variable");
No need to concatenate here to insert variables.
There's nothing here explaining the use of multiple variables, so I'll chuck it in just incase someone needs it in the future.
You need to concatenate multiple variables:
header('Location: http://linkhere.com?var1='.$var1.'&var2='.$var2.'&var3'.$var3);