As others have said I don't think you can make it go behind (as in a layer underneath) a sticky header, but you can keep it visually lower in the page by absolutely positioning it to the header so it never gets the chance to ride up over the header.
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{anchor-name: --stickyHeader;}
On the popup:
{
position: absolute;
position-anchor: --stickyHeader;
margin: 0;
inset: auto;
top: anchor(bottom);
left: anchor(left);
}