OK, like you, I use BibTeX (as seen in your screen grab). The reason @talazem’s code you quote isn’t working is that when I look at the $ReferenceRIS data there is no “U1”. Note also, his code is case sensitive, though I don’t think that is the issue here.
The result makes me assume the Cmd+Opt drag contains RIS info regardless of the formatted version that arrives into the Tinderbox note $Text. The latter depends on the current output format set in Bookends. Sorry, it’s all a bit complex, but it leads to an ‘aha!’ moment. If I edit my ‘RIS.fmt’ [sic] in the Bookends Format Manager (via ‘Biblio’ menu) to include the RIS ‘U1’ field (it doesn’t by default), then even though my drag inserts BibTeX format data, the $ReferenceRIS value now includes the ‘U1’ RIS key and the example code above works.
The edit I made, to the end of the RIS code in the format manager was this extra line:
... $UR - $z `ER - `¬
$UR - $z $U1 - $u1 `ER - `¬
Note the extra line. Note too, i just tested by editing one reference type “Journal article”. To implement this properly you will need to make a similar edit to the “Book”, “Book chapter” and “Edited book” reference typed in the RIS.fmt.
You could as Bookends’s if they can tweak the defaults for the RIS.fmt to include the ‘U1’ field but that might affect other users. So this might just need to be a thing you do yourself.
I did skim @talazem’s updated demo file but I didn’t find this nuanced aspect there (I may have missed it!).