Launch Lightbox on page load?

I have a form in a lightbox, normally launched by a su_button. How can I send a direct link that will open the lightbox upon page load? I tried using the page URL with the lightbox_content id (#signup-form), but that didn’t work.

Ex: `[su_lightbox type=”inline” src=”#signup-form” ajax=“true”]
[su_button url=”[current page url]” ajax=“true” style=”ghost” background=”#219ebc” color=”#219ebc” size=”5″ center=”yes” radius=”round” desc=”RSVP to receive the Zoom link” rel=”lightbox”]Save Your Spot[/su_button]
[/su_lightbox]

[su_lightbox_content id=”signup-form” ajax=“true]

[gravityform id=”34″ title=”false” ajax=“true”]

[/su_lightbox_content]`