Plz help me, connect a common wire for smart thermostat in furnace
I think you are right. From what I remember once I did something similar as their was no c to the thermostat. Luckily for me the unit was right next to the thermostat so it wasn’t hard to run.

You could use this instead if you don’t want to run the cable?

Venstar ACC0410 Add-A-Wire Kit https://www.amazon.com/dp/B008PLWT8C/ref=cm_sw_r_cp_api_i_OFbeDbAE61VDQ