Sorry but we do not support user modifications in code. I gave you a non standard method of using this. A regular user would never find this and it requires some knowledge.
It works totally fine with images and text. if you are using a complicated iframe then it could be troublesome.
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[button title="Button" link="#you-custom-id" target="_blank" class="" download="" rel="lightbox" onclick=""]
<div id="you-custom-id" style="display:none; max-width:90%; width:90%; height:90%;">