Homogeneous Neumann Boundary condition

Hello everyone,
I was trying to implement in FreeFem the variational formulation for the biharmonic equation in H^2_0(omega) (see Theorem 6.8-3 of P. G. Ciarlet - Linear and Nonlinear Functional Analysis with Applications, Philadelphia, 2013).

I searched for the answer in the previously opened topics but I could not find anything.
Could you please help me solve the problem described below?

I mesh my domain (say the unit disk) using a Hsieh-Clough-Torcher element (Element_HCT).
The bilinear and linear forms are very easy to write. What I cannot implement is the homogeneous Neumann boundary condition along the entire boundary.

In this case, indeed, the strategy making use of the command int1d(Th,borderLabel)(dnu*testFunc), where dun is a prescribed function and testFunc is a generic test function, is not implementable since we always have dnu=0. How would you solve this?

Many thanks in advance and best regards,

Paolo