Magic keyboard connection issue

Just purchased a pre - owned IMAC 2011. It was factory reset by the seller. The magic keyboard is the older version; it has no cable connectivity.

The keyboard works fine on my other Mac devices but I cannot get it to pair with my IMAC. The difficulty is that I still have to resister and set up my IMAC, which means that I have no access to the settings menu.

I will greatly appreciate some advice