One easy way, but might not be the best is to use the property node. right click block diagram, programming, applications control, property node.
Then right click the top(yellow) box of your property node, navigate to select class, VI server, VI, VI.
Then left click the lower box, 'choose front panel window', 'is frontmost'.
You can play with the other options on the property node to your satisfaction.
Another method is to use the invoke node which is similar to the above, but you place the "invoke node" on the block diagram instead of the property node.
Like I said, there might be other more straightforward approach.
