Check the Scripting documentation, please!
code:
<Control xsi:type="CheckBoxControl" Id="ChkThing">
<Position Left="10" Top="10" Width="100" Height="15">
<Caption>Thing</Caption>
</Control>
code:
//Top of script
var active = false;
//In BtnSave block
active = PlusWnd.Button_IsChecked("ChkThing");
//In your script block (where it should test if it's active to continue)
if(active) {
//Do it!
} else {
//Don't do it!
}