Given a smooth function, does there exist a (semi-)Riemannian metric for which it is harmonic?

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To be precise: given $\phi \in C^{\infty}(M)$, does there exist a (semi-)Riemannian metric $g$ such that $\Delta_{g}\phi = 0$?

I avoided fixing a coordinate system on purpose here; I'm not sure how that would come into play, but I expect it might.