Try a helper question to calculate the watermark text
question name: helper calculation:
concat("Taken on: @[datetime:d MMM yyyy HH:mm] ,by:", ${person}," &color=yellow&outlineColor=black&haloColor=white&size=32&bold=true&margin=10'")
and then in your bind::esri.paramters column on your image question
bottomRightWatermark=${helper}