Create Sub-Domain
BackForm-A sub-domain: mounted under a layer, competing for that layer's
traffic via a layer_slot. (Form-B sub-domains — mounted on an experiment
group arm — are created from the experiment group editor, see Layer detail.)
Form-A sub-domain: mounted under a layer, competing for that layer's
traffic via a layer_slot. (Form-B sub-domains — mounted on an experiment
group arm — are created from the experiment group editor, see Layer detail.)