Note that this must not be closer than six feet.

You can NOT bootleg the neutral or ground from the 50A spa branch circuit for a convenience outlet circuit. Either run a full separate branch circuit or make your "spa box" a subpanel and put the branch circuit breaker there.

Is this an indoor or outdoor spa?

Makes little difference if we're talking about a residence. While the rules waggle around a bit, both require the convenience outlet.
Outdoor 680.2(A)(3)
Indoor 680.43(A)

Thanks ronnatalie, I will run the additional #12 conductors back to the main panel.

petey racer, it will be outdoor.

still curious as to why my original thought of "bootlegging" the neutral and ground is unsafe or wrong. not arguing, just asking.

Thanks again

It's wrong because the code says it is.

It's technically possible to overload the neutral, and your ground may not be sufficiently safe.

One thing you should check, is whether there's any requirement to run a neutral to the spa at all. Most of the ones I've seen (including all three I've installed), just use 240 (two hots) and a ground.

The information I've got says it's a 120/240V hot tub and that it needs all four wires. It has a 1/15 hp circulation pump and an ozonator that I assume are both 120V and the heater and jet pump are 240V, I haven't seen a wiring diagram of the unit beyond the connection box yet. There is also an air blower and light.
As I type this I realize there are probably enough 120V components, that they would pull 120 from both phases to help balance the neutral load. I originally assumed it would draw 120 from only one phase and I was going to connect my convenience outlet to the other.

What model spa? Mine has a 240V circulator pump (it's still very low number of watts). The lighting is almost always 12V stuff anyhow.

Wind River, their Virtue model.

windriverspas.com

Yep, you're right. That spa does use the neutral.