Show on user click not working - incorrect element selector?

I followed the instructions in this topic to try to set up the Show on user click add-on, but I've been unsuccessful so far:
https://premium.wpmudev.org/forums/topic/popover-plugin-specify-onclick-element

The classes assigned to the button are as follows:
<a href="#" class="sign_up sign_up_green radius3 split_test" id="split_test">Sign up!</a>

Based on the topic, I understood that a#split_test should be entered into the Element selector box, but it does not work. I've tried so many combinations in the Element selector box that I've lost track.

Any suggestions are appreciated.