Same as you would for any PLC in Portal. You click the simulate button, then download to it. Portal opens up the new PLCSIM for 1200/1500, and the old PLCSIM for 300/400. Portal essentially freezes while PLCSIM is starting up, and then it will ask you to download to it. When doing the download, you need to select which PLCSIM is your target for the download method.
Note, PLCSIM may not be installed by default, so you might have to install it manually. It should be on the S7 Pro disc though.