I have tried what seems like all permutations possible to make

& Apt.backportSuite  "Stable "jessie""

work (square brackets, normal brackets, escape, combinations of all that...) but no success so far.

It seems there should go a string after backportSuite, which according to what I read should be double-quoted. However, the release also needs to be double-quoted.

What is the right way of doing this?