After creating the segment, On platform products will be the first screen to complete.
Click on the On platform product you would like to add to the segment:
Once selected, click the arrow to move the platform over to Platforms in Segment
Multiple platforms can be moved at once. Click on the first platform, type CTRL A to select all the platforms in the list.
If you don't wish to move all platforms, click the first platform, hold CTRL and click the remaining platforms you would like to move.
Once done, move onto Central Investment Proposition - Adding Off Platforms to a Segment