Using "Shape Check" Utility

I know how to open the "Shape Check" utility... but I don't know how to tell it to select a spline to check.

I'm trying to make an interface where "Shape Check" is just a checkbox where it can turn the utility on and have the currently selected spline as the parameter. When unchecked, it simply closes the utility.

So far I can open and close it...

checkbox chk "shape check"
on chk changed theState do
if theState == true do utilityPanel.OpenUtility Shape_Check

if theState == false do utilityPanel.CloseUtility()

How do I take the $ as the parameter for Shape_Check?