I guess you're not after the current post/page ID, but rather trying to find the ID for some url from your site.

Then you should try out the handy [`url_to_postid()`][1] WordPress core function.

It returns the post/page ID if it exists, else `0`.

**Example:**

    echo url_to_postid( 'http://example.com/2014/11/14/hello-world' );
  
    // Outputs the post ID as 1 in this example.

**Update:**

*Reply to the comments:* You could create a custom field, called e.g. `url_offer`, from the backend 

![Example for a custom field][2]

and use:

    $url_offer = get_post_meta( get_the_ID() , 'url_offer', true );

to retrieve it on the front end.
 


  [1]: http://codex.wordpress.org/Function_Reference/url_to_postid
  [2]: https://i.sstatic.net/39ORA.jpg