This option is not available in the (new) Basic Publishing Dialog and requires using the (old) classic publishing dialog.
The Puzzle / Study Mode lets us create problems where your readers can try to solve it by entering moves for the winning side. This example is White to Move (start off by clicking e6 and e8 to move the White Rook to e8 -- the Viewer will then make Black's move):
The non default values specified are:
Width: 500 x Height: 500
Puzzle/Study Mode = Yes
FEN = 3q3k/pp3r1r/4R1Q1/2p1P2p/Pb6/1P1P3R/1B4PP/7K w KQkq - 0 1 (cut-n-pasted from the PGN file).
Board Only = Yes
The FEN string allows the Viewer to show the initial position before loading the PGN file (when Click to Play is pressed). It is not required but it shows your users the position before the viewer is started. Without the FEN we would just see the normal initial position and then when the Viewer is started it would change to the FEN position in the PGN file.
Here is the PGN file used for this problem: