[Defender] Masking Login URL

I'm having a hard time activating the login URL masking feature. No matter what I try, I can't seem to get past the 'invalid URL' error when I try to change the standard path to anything other than wp-login.php. I'm sure I'm just doing something wrong, but the documentation didn't really provide an answer. I tried it with the full path & just the page name, but nothing works. Can you point me to some step-by-step instructions perhaps & maybe then I can see what I'm missing? Thank you.