The standard way I've seen to do momentary push buttons on an HMI is to set a bit when the button is pushed, and then reset it when the button is released. The value is true while the operator presses the button, and is changed to false when it is no longer pressed. If you need to send an integer instead of a bit, same concept applies: Press change value to 1, release change value to 0.
If you need it to be truly momentary edge detection, true for only one cycle, then rQx's solution seems like the best solution. Have the HMI send a value, and then let the PLC do it's edge detection and reset the value from the code.