Find Iso surface to impose BC

Hi fellow FreeFEM’er,
I have a 3D mesh on which I solve a PDE (time=i), for the solution in the next timestep i+1, I need to impose additional boundary conditions on the iso surface through the 3d volume (time: i).
How can this be handeled efficiently? (and preferably with methods supported by FreeFEM)

Thank your for your help in advance,

(I asked a similar question some time ago, but nobody could help me - maybe you can now? Extract Iso Surface in 3D)

Without math formulation No idea,

define a func f(x,y,z), and on the boundry, u=f.
I don’t know if it help, maybe you could try