CustomIndexNames is an option of FCCanonicalizeDummyIndices. It allows to specify custom names for canonicalized dummy indices of custom index heads.
Overview, FCFAConvert, FCCanonicalizeDummyIndices, LorentzIndexNames, CartesianIndexNames.
ex = T1[MyIndex2[a], MyIndex1[b]] v1[MyIndex1[b]] v2[MyIndex2[a]] +
T1[MyIndex2[c], MyIndex1[f]] v1[MyIndex1[f]] v2[MyIndex2[c]]\text{v2}(\text{MyIndex2}(a)) \;\text{v1}(\text{MyIndex1}(b)) \;\text{T1}(\text{MyIndex2}(a),\text{MyIndex1}(b))+\text{v2}(\text{MyIndex2}(c)) \;\text{v1}(\text{MyIndex1}(f)) \;\text{T1}(\text{MyIndex2}(c),\text{MyIndex1}(f))
FCCanonicalizeDummyIndices[ex , Head -> {MyIndex1, MyIndex2},
CustomIndexNames -> {{MyIndex1, {i1}}, {MyIndex2, {i2}}}]2 \;\text{v1}(\text{MyIndex1}(\text{i1})) \;\text{v2}(\text{MyIndex2}(\text{i2})) \;\text{T1}(\text{MyIndex2}(\text{i2}),\text{MyIndex1}(\text{i1}))