Phrases telling the reader to do something in which the first word is a verb and the second is a preposition are not hyphenated.
- Sign in at the registration table.
- Plug in your device.
They are hyphenated when they become adjectives or nouns.
- Sign-in desk.
(adjective/sign-in) - Plug-in software.
(adjective/plug-in) - Download our latest plug-in.
(noun/plug-in)
