Trigger a lightbox to automatically show when the page is loaded, and then use a cookie implementation to record when the user dismisses the lightbox so that the pop up doesn’t show again.

FIRST ADD A LIGHTBOX WIDGET AND THEN AN EMBED AFTERWARDS

The lightbox can have whatever markup/content you would like, but make sure the sizing and everything is correct.

Next drop the embed code into the code and inspect the element of the lightbox to find the a similar id for the lightbox as:
#uxi_widget_lightbox2-2-lightbox-content
and replace the lightbox referances in the embed code.

Double check the lightbox for opening in an incognito window and you can use the embed button code below as well to manually remove the cookie and recheck if there are issues.