First of all you need to create a link to a page or image using a lightbox as the target as normal.
This can be a hidden link eg: a small box shape the same colour as your background, or transparent with no line, or a link that can be clicked again if the user closes the Lightbox and wants to reopen it.
Ok, you have a small box on your page  right click it and click ‘ID’, give it a name such as 'popitup' and change the 'Write ID for this object' to 'Yes'.
Create a hyperlink on this shape as usual, to a picture or page and select your lightbox as usual.
Then create an HTML box and paste the following to the HEAD.
Next right click just somewhere off the page and select 'Attach HTML' the whole HTML for the page should now appear.
Scroll down to the <body> tag and you should see something like:
Replace the '__AddCode="PageInBodyTag"' with
Replace the name ‘popitup’ with whatever ID you gave your shape.

That’s it Job Done! Lightbox open on page load.