"I am Ms. Pöllönen."
Translation:Olen rouva Pöllönen.
Nope. Miss is neiti, and Ms. is a bit like Mr. but for women. For Ms., it doesn't really matter if the person is married or not. I'd recommend using this especially with people you don't know too well.
I don't think we have an equivalent in Finnish, though I guess they both (rouva and neiti) are sometimes used regardless of the marital status and only basing the term on a guesstimate of the person's age. Neiti for younger, rouva for older. Some (myself included) may find this a little offensive and even insulting, and it might be better to go without these altogether whenever possible.
In English the neutral term might be "Ms", but in Finnish the more neutral term is "rouva". If you are addressing the president, a court judge, the speaker of parliament or a superior military officer for instance, you will always say "rouva" and never "neiti" as in "Ms".
You also wouldn't address a court judge or a superior military officer as Ms., either, but with their proper title or rank, and you would address the president as Madam President, so this may not be the best example. Rouva doubles as address for married women and as respectful address for female officials, regardless of marital status. Ms. does not have this double function. That is why rouva should not be considered the Finnish equivalent for Ms.
Miss is pronounced /ˈmɪs/, while ms. is pronounced normally /ˈmɪz/, but also /məz/, or /məs/ when unstressed (source:Wikipedia). So they sound pretty similar, but not the same. It's understandable to think they're the same, if one doesn't know the difference. Ms. is the nearest equivalent to Mr., since it can be used for all women regardless of their marital status, and it comes from earlier address form Mistress (as do the other female honorifics Miss and Mrs). Mistress was historically used to address women with the implication "lady of the house".
Both are correct. "Olen" is the present first person conjugation of the verb "olla" ("to be"), meaning it already means "I am". In everyday speech both "Minä olen" and "Olen" are used.
If you want to emphasize (in spoken language) that I am Mrs. Pöllönen, meaning specifically me, I, and not somebody else, then you would definitely say "MINÄ olen rouva Pöllönen", with a strong emphasis on the "minä".