What's the imperative form for "to be" in Portuguese of Portugal? For example, if I want to say "Be calm my friend".
Your question is both easier and harder than it appears.
The easy part is that Portugal uses “familiar” second-person tu imperatives with friends and children, which in affirmative commands preserve the stem vowel of the verb. These are unlike the third-person imperatives one would use with você or o senhor with people you’re being more polite with for whatever reason, which being third-person imperatives instead of second-person ones would have to swap the stem vowel the way a present-subjunctive form does. (Most of Brazil uses only você forms, not tu forms the way Portugal does.)
The hard part is that you might be tempted merely to look up the second-person imperative form of be and use that. Then you’d have to think about whether this is more of a “stative” be that would therefore require a form of the verb estar as in Está quieto for be quiet, versus whether this is instead more of an “essence” be that would therefore require a form of the verb ser like a mother telling her son to be a good boy by saying Sê bom!
But you’d still be barking up the wrong tree as they say in English, because be calm my friend, or just plain settle down, could easily call for using a different verb altogether: ficar. Even though it’s a common verb, it still isn’t one most learners would think of using for be because they’d be translating too literally. So for your phrase, I would just say Fica tranquilo, meu amigo for a guy or Fica tranquila, minha amiga for a girl.
There are various other possible ways of saying this, but those are the forms you need to know about for what you’ve asked about.