You do this by using the loopback function of TMFX (look for "Recording a Submix - Loopback" in manual)
Basically you select an unused submix in TMFX, mix the signal(s) you want to record (e.g. raise the input 1 volume from the top row, and fx return from the submix on the bottom row) then turn on Loopback from the submix settings (the wrench icon).
When you turn the loopback on, then instead of getting the (pre fx) signal directly from inputs 1/2 to logic, logic will now be recording the signal from TMFX output/submix AN1/2! If you want, you can keep recording the dry input 1 to logic (with loopback off), and record the wet fx signal to a different stereo track on logic to be able to adjust the fx level later. Simply reset one unused submix in totalmix, turn fx return up and loopback on on that submix, and record that submix with logic to a different stereo track.
Fireface UFX+ | Fireface UFX | Babyface Pro | 12Mic