I do not know anybody that consistently attracts indigo buntings to their feeders. The only times I have seen them feeding in my backyard has been in the spring. In each case, the beautiful all blue birds fed on white millet seeds offered in open trays or scattered on the ground.
With that in mind, if you want to try to attract this spectacular bird to your backyard for the first time, I suggest you begin making white millet available to any indigos passing through your area.
If you are successful, the first birds you see will be males. Female indigo buntings migrate north a bit later.