onclick command

I am trying to use a popup plugin that uses the onclick command to launch the popup on a button press.

When I try and use the onclick for an image, the onclick portion of the link isn’t saved in the text block. Here is what I try (which is exactly how it is instructed for the plugin):

but when I save the text block, or even switch from text view to visual view, then switch back to text view, the whole onclick is remove, like this:

What am I missing?

Please help!