You need to control the keyframes on the layer where you added your new symbol (I assuume this is above to layer you were provided). Use a keyframe to display your symbol when you want it superimposed and another KF to remove the symbol when you do not want it displayed. You can trigger the gotoAndPlay or gotoAndStop methods using button or other events to determine which keyframe to play next.
I've added a new layer and placed it above the existing layer on the timeline. I inserted a Key Frame as you mentioned, and then dragged the RegForm symbol onto the main stage. However, when I test the application, I get a flickering RegForm in the background of the existing layer.
What settings to I adjust to prevent the flickering, load the RegForm layer first and then load the second layer after RegForm Submit button is clicked?
If you are flickering, you are looping through the timeline. If you do not declare a stop, the processor immediately loops from the last to the first frame once it completes the last frame. Place a stop() action on the keyframe where you wish to stop processing the timeline.