Thanks for the answer.
> grep -A1 -B2 xsel doc/sample.nanorc
Not sure if this is just my copy of GNU nano, but sample.nanorc is actually
sample.nanorc.in.
># bind Sh-M-T "{execute}|xsel -ib{enter}{undo}" main
Just to note for anybody who reads this exchange in the future, xsel is 3rd party code.
If this feature doesn't work out of the box, check to see if it's installed.