How to set boundary conditions where u multiplied by the boundary normal vector or u multiplied by the boundary tangent vector equals 0?

Hello everyone. I have the following question: u is a 2D vector, and I need to impose the boundary conditions un=0 or ut=0,
where n and t are the boundary normal and tangential vectors, respectively. I couldn’t find the relevant syntax in the FreeFem documentation. How should I write this in FreeFem?