How to get url path info into a form variable
How to store the data depends on what you want to achieve with that data or how you want to process it afterwards. The easiest solution for that is probably using watchdog() after form submission:

watchdog('myblock', $url);


To get the current path there is current_path():

$url=current_path();


Please beware that you must disable caching for that block to have the code in your block processed all the time. Here is an answer on how to create a block programmatically in Drupal.