Getting back a permalink from custom url

okay i have made my own rewrite rule, with the add_rewrite_rule function in wordpress to get a custom url, for an plugin page.

This works great and i can visit the page [domainname.com]/login to visit the page. Now the problem how do i best get a propper permalink for this page, one that works both with rewrite rules turned on, and off.

I tried using the get_permalink() function, and that works with pages, post on other stuff, but is there a function working in similar fashion that can get custom url’s like this?