How can a p-polarized light be defined in the User Defined port?

Hi,

I want to define a TM light propagating perpendicularly in +y direction in a "User Defined" port inside a medium with periodic BCs in both sides. My inplane components are x-y and the z direction is perpendicular to the 2D simulation plane.
I should define the input quantity as Magnetic field in z direction, I wanted to know the equation I should insert in magnetic mode field bar. Is it exp(i*(kax*x+kay*y))
And the propagation constant would be the ky component, isn't it?

Regards,
Eisoul