Desperately need middle click—iMac/Sierra 10.12.6

What is the trick to assigning a middle click function to a mouse on my 5K, 27-inch, Late 2014 iMac?

There are no alternatives for the program I'm using. I flat-out need to be able to execute a middle click with my mouse. No key combo seems to work. I've added the function on my Kensington trackball, but that is a no-go. Also doesn't work with my bluetooth Apple Magic mouse or the regular corded A1152 mouse.

I know that nothing is impossible with a Mac, so what's the secret? Is there a terminal command? Do I need to buy a special mouse? What do I do?

The program is Davinci Resolve 15, if that makes a difference.

Thank you

Computer details

4 GHz Intel Core i7

24 GB 1600 MHz DDR3

AMD Radeon R9 M290X 2048 MB