How to get a Portuguese NIF for Non-Residents

I still don’t know exactly what I need to do. I am a Japanese citizen with US permanent residency. Will you bring tax rep with you in case they might ask? Tax rep I contacted in Porto asking for 350 euros to go to the office with me. I would love to save 350 euros if it is not mandatory. Should I find any residence who willingly go with me? I will definitely compensate.