Follow these instructions: What to do before you sell, give away, or trade in your Mac ...
Restarting your Mac with those keys held down ought to work. To do that read start up from macOS Recovery.
What happens when you try? Be sure to follow Steps 1 and 2 explicitly. The addition of the Option key is... optional. It merely forces a connection to Apple's recovery servers vs. your Mac's internal Recovery Partition. You only need to add the Option key to the ⌘ r key chord if that internal Recovery Partition is absent or damaged, and in that case your Mac will attempt to connect to Apple on its own anyway.