First time setting up iMac, my wireless keyboard is not detected, so I have no way of getting into the Bluetooth settings

My old iMac from 2013 just came back from repair as my hard drive had previously crashed. I now have a new hard drive with the latest possible system software installed on the old machine (they could not tell me which system it is at the repair shop). The problem is: my wireless keyboard does not connect. I know that I am supposed to go into Bluetooth settings in order to connect. However, I cannot do this, as I am not able to go beyond the first steps of setting up my iMac! It asks me to create an account and I cannot get beyond this step without typing in information. Interestingly enough, an ergonomic mouse from a no-name brand works just fine (with Bluetooth connected via the USB port on the back of my iMac).