negate boolean variable

i have a Boolean variable stop_auto_assignment and want to send the
opposite value in the request body. so if the variable value is FALSE i
want to send TRUE and vice versa. any suggestions?
i already tried to set exclamation mark before calling the variable but it
failed

this is a part of a body of request:

{
    "max_orders_assigned": 5,
    "stop_auto_clock_out": false,
    "max_threshold_waiting_time": 3600,
    "interval_weight": 4.0,
    "stop_auto_assignment": ${stop_auto_assignment},