Select the surface you want to assign the boundary and then right click it and go to assign boundary option and select the boundary of your choice.
I do not think that it was question but your comment will give me better understanding about your problem lol
ok..
well i have one more question.
should we assign Excitations like waveport on PML boundary...means when we apply PML its creates boxes surrounding our model on these faces exicitations are works or not?
waiting for your kind response.
Thanking
As per my knowledge you can not assign boundary and excitation on the same surface of geometry as it will result in error. By the way why you need to define the boundary and excitation at the same place?