Optimize the constant by minimizing the error whit no direct relation

I want to find the optimised value for the three constant C1, C2, C3 by minimising the error of ErFEM.

I run an ABAQUS job; in the first job, I define C1, C2, C3, and the ABAQUS generates a result, let's name it FEMresutl.

I subtract the FEMresult from the gold standard, and the final result is ErFEM.

ErFEM should become less than 50um by optimising C1, C2, C3.

However, there is no direct relation between the ErFEM and C1, C2, and C3.

How can I write an optimisation code that optimises the C1, C2, C3 by minimising the ErFEM?

Thanks