All answers I’ve responded with refer strictly with v9 themes, totally different ball of wax.
My bad for assuming we were talking about v9. It may actually be a little easier to accomplish since you are using v8.
Note:, it’s always a good idea to mention the version with the problem, the child theme being used and post an URL if applicable. The more information on a problem, or to replicate the problem, the better.
Disregard the previous solution mentioned., however if you ever need to use the /CHILD_THEME/_functions.php file then it would need to be created if it doesn’t exist.
/MJ/page-login.php is the file for version 8.
Copy the file into the child theme directory and that’s where you’ll make changes.
A couple questions:
Are you using a child theme? if so, which one?
If applicable, what is the directory location you’re currently placing the custom files?