AFAIK you have to pair using a usb cable, but my macbook has only USB-C cable and I don't have the adapter with me.
Is there a workaround?
If you have a compatible device you can connect it using Bluetooth in System Preferences -> Bluetooth.
If you haven’t set up your devices:
Connect a Lightning to USB Cable1 or a USB-C to Lightning Cable to the Lightning port on the device2 and connect the other end of the cable to your Mac.
Make sure that the device's power switch is in the ON position. (Green is visible under the switch when the device is on.)
Choose Apple () menu > System Preferences, then click Bluetooth to open the Bluetooth preferences window. When the device pairs with your Mac, it appears in the list of devices.
Check the Bluetooth preferences window to see your device's charge level. When the device is charged, unplug it for wireless use.
This does not require a USB cable.
To avoid connecting another persons keyboard you can rename yours. To do this control-click the keyboard and click rename:
Then you can change the keyboard name.
I don't know if this was because of lower battery or just that it needed to be plugged in, but simply by connecting a charger (from the wall) to my keyboard and then going to MacBook's Bluetooth I was able to connect/pair it. This makes me wonder how you can avoid accidentally pairing another person's keyboard or potentially highjacking a few of keyboard strokes...