how can I attach a lumecube video conference light to top of my iMac?

Then you should contact the manufacturer of the product.

As shown in this picture?

MtD

I don't think I would attempt to do that as it would likely damage the display. You may want to consider using a small tripod set up next to the iMac. Something like:

https://www.amazon.com/Premium-Fugetek-Smartphones-Compact-Cameras/dp/B01LY7HYWU/ref=sr_1_1_sspa?crid=2VDZ0MCFIQ2EY&dchild=1&keywords=table+top+tripod+stand&qid=1606152289&s=electronics&sprefix=table+top+tripo%2Celectronics%2C252&sr=1-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEyWkJORTdIME45TVU1JmVuY3J5cHRlZElkPUEwNTU3NzU3TUFWUlNUQ05TV0RIJmVuY3J5cHRlZEFkSWQ9QTA4NjMzMDcySTJFMjhKTkFFSFVDJndpZGdldE5hbWU9c3BfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

Other than that I'd consult Lumecube for support on their product.