Most electric fireplaces come pre-wired with a 72" (1.8M) cord and a grounded plug. For planning, see if your electrician prefers the line on the unit's left or right side.
Depending on circumstances, some fireplaces simply plug into a wall outlet. The only issue with plugging into an existing wall outlet is the possibility of something else on the same circuit and tripping the breaker.
You can avoid this by running a dedicated circuit from the electrical panel that only serves the fireplace. Most installations use a dedicated circuit. We highly recommend it.