HFSS Transient for modeling THz pusled Imaging

I am new to HFSS and I need help with simulating focused gaussian THz beam in HFSS transient. The Gaussian beam option cannot be selected as an incident wave in HFSS transient solution type(I don't know why). I actually want to get the reflected signal from different points on a sample when excited with a Terahertz pulse (That is is why I want to use a focused gaussain beam). Please if you have any ideas on how this can be done in HFSS, i will appreciate it.