Levelset Dirichlet Condition in 3d on phi=0 surface

Dear All,
I hope you can help me with the following problem.

I have a mesh (3d) where there is a rather sharp levelset function “phi” defined by scalar values (threshold).
I have found the method int2d(Th,levelset=phi) … respectively the 3d variant (integrating over the volume where phi == -1)

Is there a way to impose a Dirichlet boundary condition on the levelset=0 function in 3d (on the 2d iso surface where phi=0 ?)?

The usual work around with a varf and accessing the Stiffnesmatrix and Lastvector A(ii,ii)=tgv and b(ii)=tgv*bcvalue does not work because I can’t find the zero boundary since on none of the nodes it is exactly zero.

Can someone give me a hint on how to solve this?

Best regards,