You need an account with a registered product; That generates a new key. Old keys would not generally work as they would be associated with an already registered product. I don't think they are recyclable.
Keys are "redeemed" by entering them in the registration screen of the app, after the app is installed. So no, you cannot redeem the key before you download it.
So, as @CubeAce (Ray) said, you need to install the Trial version and then try entering the key. However, since your key is apparently for MEP 2018, you really need the installer for that, now old, version in order to be able to properly test this. You might need to ask Magix Sales Support if they would give you a download of that version.
Keys are "redeemed" by entering them in the registration screen of the app, after the app is installed. So no, you cannot redeem the key before you download it.
So, as @CubeAce (Ray) said, you need to install the Trial version and then try entering the key. However, since your key is apparently for MEP 2018, you really need the installer for that, now old, version in order to be able to properly test this. You might need to ask Magix Sales Support if they would give you a download of that version.
Both Ray and I have told you that you need to download the Trial version of MEP 2018. You will need to contact Magix Sales via the Support button in the header of any forum page, explain what you are wanting to do and ask them to give you access to the download. Once installed, you will be asked to enter a registration/serial key, that is where you should enter the key you have.
As a matter of interest, since the key is, apparently, for the 2018 version of MEP, why have you not used it before?