How to control part cooling fan on octopus PRO?

can somebody help me with part cooling fan control wire... 2 years ago(don't get me started on why only now i'm building it. (worst diy project i've ever attempted to do in 32 years of my life)) i was sent the octopus PRO... not the octopus v1.1 as in all the fucking documentation...
so now i don't know where to connect that third wire

I googled for over an hour in vain. so my fan is now either 100% or 0%

I found this: https://os.ratrig.com/docs/guides/4028/
but it's so vaguely written, what pin number is it? etc...
4028 server fans are becoming a popular option for part cooling because of their light weight and great pressure and flow rates. In this guide we'll go over how to connect one and use it for part cooling. 4-pin fans usually aren't used in 3d printers, and many boards only provide 2-pin ports, but fear not! There's always a way.
Connecting a 4028 fan | RatOS
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