i dont think you can; you need to control the private key of an address to sign a mesage from it, which you cannot do with a xapo address. fact stll remains you tried to lie by signing form that other address, im going to say this is a hacked account. youre coming up with excuses at this point.

that aside, your address you posted in the op: 3DxDzNcQLw1uTRs8mDMr117b6twycCQ7wj
is a multisig address, maybe its linked to someone's bit-x account?
someone get on that.