“PR” means pull request. Where you submit a proposed change to the code and wait for someone with control over that code base to accept it.
You could also fork the plugin, which you can look up yourself.
Here’s an example of something that handles a BBcode. You’d need to add something similar for whatever you want to happen for your login:
You’d change “small” to “login” and do something about the style, and maybe need to add some CSS somewhere to make it do what you want.