Oh, good! No, if you pick that option from the ones behind the little flag, that's exactly what is needed.
That way, the volunteer contributor team get a message about it and see what you wrote. If they decide your answer was indeed correct, they add it to the database of possible answers.
And if you have your notification emails from Duolingo on, you get an email thanking you when that happens -- how cool is that? Then you know that you've helped improve the course. This may take a long time, though, I've sometimes received these months or even years later...
'Tämä' is in the nominative case; it is used for subjects. So 'Tämä on mikrofoni': 'This is a microphone'.
'Tässä' is in the inessive case. A couple translations of it would be 'in this (place)', or 'right here'.
Therefore you might expect 'Tässä on mikrofonia' to translate to 'In this is a microphone', or 'Here is a microphone'. However, a noun in the inessive case, plus the verb 'on', is used to translate the English verb 'has'. So 'Tässä on mikrofonia': 'This has a microphone'.
I admit, 'This (thing) has a microphone' and 'In this (thing) is a microphone' mean basically the same thing to me.
Note, translating 'has' with the inessive case is only done when the inessive noun is a thing, not a person or animal. A person or animal would instead take the adessive case ending -lla or -llä. So 'Hiljalla on mikrofonia': 'Hilja has a microphone'.