Ok, right!
In principle, if you've done it all right, you can leave them at default. That is, if your atoms are ordered in the same way in the central region and the electrode, then the general rule applies: the first atom in the left electrode is equivalent to the first in the central region, and the last in the central region is equivalent to the last in the right electrode. That is,
equivalent_atoms = [[0,0],[-1,-1]]
which is the default.