You must have front derailleur cable housing that runs along the top tube and then down the seat tube and the housing itself pushes on the front derailleur's operating arm while the inner cable is held stationary. This is often done on CX and MTBs to keep the cable from running under the bottom bracket shell. You would have to reroute the derailleur cable along the down tube and under the bottom bracket shell and then up to the front derailleur as is common with road frames.