Machine Prefix is not applicable to XaaS requests, please refer to the documentation
https://docs.vmware.com/en/vRealize-Automation/7.3/com.vmware.vra.prepare.use.doc/GUID-CDB547F8-0529...
Alternative method:
Using XaaS form to pull the prefix off the Business Group that submits the request. Here are the details:
1. Call the "getProvisioningGroupEntityByName" method in the "com.vmware.library.vcac" module, pass in a vCACHost and Business Group name (obtained from request properties). This returns an "Entity" object.
2. Call the "vCACCAFEEntitiesFinder.getBusinessGroup" function, passing in a CAFEHost and the id of the Business Group Entity object (group.getProperty("GroupID")). This returns a vCACCAFEBusionessGroup.
3. Call the "vCACCAFEEntitiesFinder.getMachinePrefix" function, again passing the CAFEHost and the Machine Prefix ID (cafegroup.getMachinePrefixId().toString()).
Then you can use the vCACCAFEMachinePrefix object returned in step 3 to obtain the prefix... "objPrefix.prefix". Sorry if this is off base and doesn't apply to your situation.
Sincerely,
Rahul Parmar
VMware Support Moderator