Slot Antenna Port Assignment!

If I am feeding a slot antenna with a microstrip feeding line, how do you make the exact assignment of the waveguide port?

Configuration of the antenna:
Top Layer has a slot
Middle is the substrate
Bottom Layer has a microstrip feeding line of width equivalent to 50 ohms

Radiating side of the antenna is the slot.
Also if someone can comment on the boundary conditions for this type of antenna?

Looking forward to your suggestions.
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