Transferring messages from a motor neuron to a leg muscle requires the neurotransmitter known as

Acetycholine (Ach)

Explanation:

Acetylcholine is an excitatory neurotransmitter that can act on both neuronal synapses and motor plates that send signals to the muscles.

A neuromuscular junction is the junction between the terminal part of a motor axon (neuron) with a motor plate (or neuromuscular synapse), which is the region of the membrane plasmtic  of a muscle fiber, where the meeting occurs. The nerve and muscle allowing to trigger muscle contraction.  For this neuromuscular junction happens, it's necessary the acetycholine.

In motor plates (neuron - muscle), receptors in muscle fibers recognize acetylcholine as a sign of contraction.