Just realized I should add this...
I have to impose a BC on the function itself and its derivative. So I have to use a Dirichlet BC.
If it were just the derivative, I could use a Neumann, but I have to impose both.
How do I set a slope-matching condition in COMSOL?
The Attempt at a Solution
I have used the Dirichlet BC type. I have multiplied and divided by n, the number of equations, on either side of the boundary.