Preserve Underlying Transparency is in the Modes column. Pressing F4 switches between Modes and Switches. There is also a button at the bottom of the timeline under these columns that says Toggle Switches/Modes. You can also click on the Expand or Collapse Layer Switches icon at the far bottom left corner of the timeline to reveal both the Switches and Modes columns. You'll see a T between the Modes and Trkmatte titles at the top of the Modes column. Under that T you'll see little square boxes. When you activate PUT the box changes to a checkerboard.