Hello, I'm seeing a T540P W540 W541 "Power Jack cable" available for less than 5 bucks (aliexpress, ebay, etc). The cable has the charging port and a connector that goes on the mobo so should be a pretty easy fix, no soldering.
Do you have the HMM (hardware maintenance manual) downloaded to double check?
Btw you don't need the whole "lenovo square tip" charger if you have a 20V capable USB C PD charger (65W or more), you can get a 3 bucks adapter, I use it all the time it works perfectly.