As a verb, use log in. Write log in to, not log into. As an adjective and noun, use login.

• As a verb: To log into in to your system, enter your credentials, and then click Submit.
• As a verb: Users who log in are not affected.
• As an adjective: You can change the text and layout of the login form.
• As a noun: This option locks out an operator after a certain number of failed logins.
• As a noun: As a best practice, automate Citrix logins.