Connecting Common Wire to Control Board
Use the terminal block below the control board

The terminal block below the control board on your system, with the brown and yellow wires connected to it, is the distribution point for 24VAC common (X on your system wiring diagram) in your system.  Simply crimp a 1/4" (6.3mm) female quick-connect suitable for 18AWG wire (red color code) onto the end of the wire you want to use and slide it onto one of the tabs on the terminal block. If you need to extend the C wire, you can use a piece of 18AWG TFFN/AWM wirenutted onto the C wire in the thermostat cable, with the quick disconnect crimped on the other end.