Hello Paulo
The issue you will have with the rectangular input field inside a dielectric waveguide is that the field profile will not match the modes of your waveguide so you will have odd propagation patterns and particularly strong evanescent fields.
When exciting a waveguide the more conventional approach would be to use the mode profile of the waveguide for excitation. This option is available to you through the modal , this is precisely what the modal option within Input Field Transvere section within the Input Plane source.
What kind of behaviour are you attempting to model? Coupling to a waveguide or propagation within the guide?
Scott