here is how HFS chooses the port
1. if a port is selected in the saved configuration and it is free, then it is used, else step 2
2. if the port 80 is free, then it is used, else step 3
3. a random free port is used
actually the only way to get a random port is to have no saved port in the configuratoin and to get the port 80 occupied.
if you just don't want the port 80 to be used, you could just set another port and save the config.
if you want an always changing port you could try to occupy port 80 with another software.
if this does not satisfy you, tell me about your situation and will think about it.