Dear Athira,
There is an Optical Gaussian Pulse Generator in OptiSystem. However, you may use the Matlab or Python components to use your desired beam characteristics and load it to OptiSystem. Example on using these two components can be found at the following link. Typically, OptiSystem requires the optical field that is passed to FSO channels.
C:UsersUSER NAMEDocumentsOptiSystem 19.0 SamplesSoftware interworking
Regards,
Ahmad