That value is a random-ish string generated for one-time use with a lifespan of, if I remember correctly, 12 hours. I am not sure what you mean by “get” the value. Assuming you mean “generate the nonce” then…
You want wp_nonce_url or one of the related functions.
wp_nonce_url( $actionurl, $action, $name );
For example:
wp_nonce_url( 'http://www.google.com');
You can’t regenerate the value if that is what you want but you could regex it out of the URL with PHP, circumstances permitting, or use Javascript to search the generated markup. Since I don’t know why you are trying to do this, a solid answer is difficult.