Cross vCenter vMotion between two different vCenter SSO Domain is NOT supported currently with the vCenter REST API. I only know this because to do so, you also need to have AuthN into destination vCenter Server and the current REST API doesn't support that which is needed.
You'll need to use the vSphere API if you wish to automate this as the REST API only supports migration within the same SSO Domain. For details on using the vSphere API, you can refer to https://williamlam.com/2017/10/when-to-use-move-vm-cmdlet-vs-xmove-ps1-script-for-performing-cross-v...
