Having been told once that I had a typo for missing the accent off the A in Siân, I copied and pasted one from elsewhere, but you shouldn't have to do that. Either we should have the accented letters available even when we're typing English or it should accept Sian without the accent.
If you're using the web version, there should be a little button underneath the text entry field with an "â" in it that you can press to make the letter.
If you're on a mobile device (phone/tablet), try long-pressing the A key to get a pop-up containing accented versions of the A.
If you're supposed to be translating into English, then I think it should accept both Siân and Sian - I think they should only expect accents when writing in Welsh.