We will label the applied push (or pull) force with F.
Tension in an object results if the pulling force acts on its ends, such as in a rope used to pull a boulder.
If we say that our carriage is being pulled by a bar (by the horse) at its front end, then we can add this force to our FBD.
And there we have it: all the forces acting on our carriage have been labelled in this figure. This is the complete FBD for our problem of a carriage being pulled along a road.