Just to clarify - you are starting this workflow by right-clicking on a virtual machine in vSphere Web Client inventory and choosing the workflow from its context menu, correct?
In this scenario, failure to pre-populate the input field means that there is some mis-configuration. When you registered vCenter endpoint in vRO using the workflow "Add a vCenter Server instance", how did you provide the vCenter endpoint address - by FQDN (eg. myvcenter.domain.com), by its short name (eg. myvcenter), or by IP (eg. 10.111.23.1)?
Please make sure your vCenter instance is registered in vRO by FQDN; otherwise, the logic that should pre-populate the field value in Web Client won't work correctly.