How to connect my Old 2012 Keyboard to my new 2019 iMac by blue tooth.

Hey there, zeetoe25.

After reviewing your post, we understand that you want to use your old wireless keyboard instead of the new one that came with your new iMac. Please follow the steps below from this article: Set up your Apple wireless mouse, keyboard, and trackpad

"How to set up earlier Apple wireless devices

If you're not sure which device you have, you can learn how to identify your Apple wireless mouse, keyboard, or trackpad. 

Then use a wired mouse or trackpad — or the built-in trackpad if you're using a Mac notebook — and follow these steps to set up your earlier Apple wireless devices (such as the Apple Wireless Keyboard, Magic Mouse, and Magic Trackpad):

1. Turn on your device.
2. Wait until your device's LED starts blinking, which means that it's in Discoverable Mode. (If the LED is on but not blinking, your device is already paired with another host. To unpair it, choose Apple menu > System Preferences, then click Bluetooth. Hover the pointer over the device you want to remove, then click the button  that appears next to the device's name. Turn the device off, then start again at step 1.)
3. Choose Apple menu > System Preferences, then click Bluetooth.
4. Wait while your Mac searches for your device. When your device appears in Bluetooth preferences, click Pair1.

1. Apple Wireless Keyboard requires the passcode that appears on your Mac. Enter the 8-digit passcode on the keyboard, then press Return. Earlier models of Apple Wireless Mouse or Apple Magic Trackpad will connect automatically or show a dialog. If you see a dialog, click Pair or press Return."

This article might be helpful a well: If your Apple wireless mouse, keyboard, or trackpad isn't working with your Mac as expected

We hope that helps. Cheers.