PDE implement Floquet boch Boundary condition

Hi,

I am working in PDE module.I need to solve Laplace Equation with single variable.My domain is a rectangular domain.I know that my solution varies sinusodially along x.I want to know how I can impose this boundary condition.I have read articles that we have to use Lagrange multipliers in weak constraints.But can I know how may I do the same.Or any other procedure? Thank You