Startup from macOS recovery without keyboard

Hi, my dad wiped his iMac for a new user to give it to me. I got it out, bought an apple trackpad and generic Bluetooth wireless keyboard. When I turn the Mac on it shows the flashing question mark and apple.com/support/startup which tells me to turn it on then immediately press 2 keys on the keyboard. However, the new keyboard says to use it I first need to pair it with Bluetooth. Which I can’t do without the computer working. As the keyboard is rechargeable, I tried plugging it in to the computer and pressing the buttons but as it’s not yet paired it didn’t work. Suggestions please!