manually attach element group to box

I have an element group that I would like to attach to a box, I know that this is possible but its very frustrating to do it through the UI (and simply wont let me sometimes).

Is there a way to manually “attach to box” and not through the drag and drop feature?

If you mean using CSS or code snippets - then no.

You would be better suited going through Wix Support about this issue as it is not a code issue:
https://support.wix.com/en/article/contacting-wix-support