How to send an iMessage to an American contact?

I live in Ireland. I'm trying to send an iMessage to my friend who lives in the states. She gave me her mobile number (her iMessage is on her iPhone) but I can't send the message! The send button is grey. Do I have to put a prefix number (001) before her mobile number or anything?

I think you have it right, 001 then area code / number. Have them send you a text then you can just reply and have it saved.

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