Question: Is it possible to create a puzzle that behaves like chess.com puzzles?

 The Fen = r2q1rk1/2p2p2/pb1p3p/3B2p1/1p1P4/3Q2P1/PP3PP1/R3R1K1 b - - 0 19

Specifically if White is solving, the puzzle starts by showing Black's move, 19. .. RB8.

For example:
19... Rb8 20. Qg6+ Kh8 21. Qxh6+ Kg8 22. Be4 f5 23. Bd5+ Rf7 24. Re6 Qf6 25. Rxf6 b3 26. Bxf7# 

I can't figure out how to have black move first and then start solving.

https://support.chess.com/article/316-how-do-i-put-a-game-or-puzzle-in-a-forum-post

@justbefair, that is a solution, but not what I was asking for. A Chess.com puzzle would begin with the move 19. .. Rb8 to start. Then the rest would follow your solution.

Also the link, https://support.chess.com/article/316-how-do-i-put-a-game-or-puzzle-in-a-forum-post,  makes no mention of the annotation dropdown entries:

I'm clueless as to their function.

Your initial FEN string has the black-to-move option set. You post the PGN with the puzzle moves added to the initial FEN. When posting, check the puzzle and flip-board checkboxes in the theme tab.

[Event "?"]
[Site "?"]
[Date "????.??.??"]
[Round "?"]
[White "?"]
[Black "?"]
[Result "1-0"]
[SetUp "1"]
[FEN "r2q1rk1/2p2p2/pb1p3p/3B2p1/1p1P4/3Q2P1/PP3PP1/R3R1K1 b - - 0 19"]

19... Rb8 20. Qg6+ Kh8 21. Qxh6+ Kg8 22. Be4 f5 23. Bd5+ Rf7 24. Re6 Qf6 25.
Rxf6 b3 26. Bxf7# 1-0

@manekapa I haven't been clear enough.  That's not what I want. The puzzle is from White's perspective, so Qg6+ is the first move of the solution. What I want is the start of the puzzle to make Black's Rb8 move and then white to move.

In chess.com puzzles the animation starts with then opponent's  move and then it is the solver's move. For example https://www.chess.com/puzzles/problem/1020332 
starts by showing White's Kb3

My guess is that one can't do it with the current system.