Unable to find an online tool to do this, but in terms of how, the following appears to be the logic that would be used:

• Both white and black pieces would be in the correct starting positions;
• White would start;
• Turns following white's start would be exchanged between white and black;
• Any piece able to move during a turn would be a member of the list of pieces to be randomly selected to move;
• Once a piece was selected, all squares for that piece that are legal moves would be added to the list of possible moves, and a move would be selected randomly from that list;
• Attacks would not take place unless the attack was randomly selected as a move. If an attack does take place, then the attacked pieced would be removed from the board.
• Iterations of randomly selected moves would stop once the number of moves into the game set before generating the board had been reached.

I was unable to find an online tool to do this, but in terms of how, the following appears to be the logic that would be used:

• Both white and black pieces would be in the correct starting positions;
• White would start;
• Turns following white's start would be exchanged between white and black;
• Any piece able to move during a turn would be a member of the list of pieces to be randomly selected to move;
• Once a piece was selected, all squares for that piece that are legal moves would be added to the list of possible moves, and a move would be selected randomly from that list;
• Attacks would not take place unless the attack was randomly selected as a move. If an attack does take place, then the attacked pieced would be removed from the board.
• Iterations of randomly selected moves would stop once the number of moves into the game set before generating the board had been reached.