Hi, so you are a bit confused. I get why.
Look on the bottom of your logitech keyboard.
See if you find this orange logo.
If you do find this orange logo, that means it supports what Logitech calls “Unifying Technology” where it easily pairs to the pictured receiver.
You can purchase one from here:
Once you purchase this USB receiver, you also need to download the pairing software.
Install the software, then plug in the USB receiver.
Run the software and follow the instructions.
It will tell you to add a device, and set the receiver into pairing mode.
To pair, simply switch off and on the keyboard.
Now if you don’t have a power switch on the keyboard, you already know that this is not a supported keyboard. List the model number of your keyboard, not the name of it if these instructions don’t work.