i havnt played too much with it but i made a skirmish to test it.
I made my route with the ferry command at the top.
i select transports and right click on the ferry initial entry point, this i believe puts em in a wait state.
I move land units to be transported to be ferry initial entry point, either just right click, or having the rally point for my factory end there.
at this point the transports automatically pick them up and take them to the destination then return to pick up more.
placing a ferry point on transports doesnt work, you have to select and move them after its placed. I recall not having success with setting the rally point for transports at the ferry point. i still had to move them once manually so they registered as ferry transports.
depending on what you're doing it might be cheaper to make the amph teleporter.
with the teleporter you can set your rally at the teleport then shift commands after theyve been teleported. it looks weird on the screen but works fine.