Did you get this as a listening exercise? (Since this sentence discussion is for a Welsh to English translation, I think the only situation where Welsh text would be expected would be a listening exercise.)
As far as I know, it's not possible to include variants for listening exercises. You have to type what the voice says... not a synonym that you yourself might use but that's pronounced differently. So Shwmae/S'mae, fo/fe, isio/eisiau etc. have to be entered the way the voice says it.
This course is designed to teach you only the Welsh that you will most likely encounter, and as "shwmae" is far more common than "siwmae", there was no point of having "siwmae" as an accepted spelling, as you might as well just learn the "shwmae" spelling. "sh" seems to be used in newer words, especially loanwords, whereas only "si" is present in old words.