Hi cesim, you can try a target expression like this:
f() = f(x_i, p_i)
However, in your original form of 0 = f(x_i, p_i), I would expect Eureqa to introduce constant terms on the right hand side which you could then manually factor to the left hand side.
e.g. 0 = a*x_i + b*p_i + c would turn into -c = a*x_i + b*p_i