This allows you to sign into Youtube on your phone.
This allows you to sign in to Youtube on your phone.
Which is correct? I think it's "sign in to" because "sign in" is a compound noun (I think that's what it is called) so you can't really mutate it by adding "to".
"Please sign in."
However, if you want to specify exactly where that should happen:
Please sign in to your account.
Please sign in on the sheet of paper at the reception desk.
Please sign in at the reception desk before you enter the building.
Ah, a phrasal verb. Thanks for the info. I've never heard of that one :)