Target Display Mode not working Late 2009 iMac to 2019 Macbook

I'm trying to use my Late 2009 iMac as a second monitor. I have a USB-C-to-Thunderbolt adapter, and a Thunderbolt cord going into the old iMac. Pressing Command-F2 does not work (both with and without the Fn key). I have tried pressing it before login (at the login screen), and after I'm logged in. Nothing.

I have read other posts that the adapter does not work for Mini DisplayPort as Mini DisplayPort and thunderbolt 2 are not the same thing. And I should use

A) USB-C to Mini DisplayPort cable instead, or

B) USB-C to Mini DisplayPort port adaptor for about $30 and use a Mini DisplayPort cable.

Before getting additional cables/adapters can anybody let me know if this works?