AFAIK, not all VM operations-based vSphere REST APIs are available in current releases. We may expect it in the next major vSphere release. But in SOAP APIs (vSphere Management APIs), have the option to set the disk provisioning type of a hard disk.
Sample code:
Add new harddisk with specified capacity and disk provisioning type for a VM:
https://developer.vmware.com/samples/1820/add-new-harddisk-with-specified-capacity-and-disk-provisio...
