Login can be one word or two, depending on its usage.
In noun form, use login.
In verb form, use log in.
Examples in noun form:
Rusty entered his login into the cat fanciers forum.
Sasha forgot her login and had to get an email reminder.
Examples in verb form:
Rusty will log in to the cat fanciers forum.
This is the eighth time Sasha tried to log in.
Remember: If it’s a noun, use one word (login). If it’s a verb, use two words (log in).