Charging magic apple keyboard when you don"t have an iPad?

According to this support document:

Set up and use Magic Keyboard for iPad - Apple Support

an iPad Magic Keyboard does not need to be charged. But it's designed to be used with (and presumably powered by) an iPad, so what are you attempting to do with it if you don't have an iPad? Or are you referring to a different Magic Keyboard?

And just a friendly tip: have a bit of patience when you ask a question. This isn't live chat and Apple Support does not participate in these forums. We're all just fellow users here, helping as we have time and knowledge to share, so it may take more than the very short time you waited before anyone even sees your question, much less has time to formulate a diagnosis and post a suggestion.

Regards.

You can charge the Magic Keyboard & Mouse using the supplied cable that came with your iMac. Just plug the cable into a port on the back of your iMac then plug the other end into your keyboard or mouse.

See: Recharge the built-in battery in your Apple keyboard, mouse, or trackpad - Apple Support