This is clearly documented for vRA7 as per https://docs.vmware.com/en/vRealize-Automation/7.1/com.vmware.vra.programming.doc/GUID-B5EB1871-458A...
But looking at the different API's available for vRA8 I cannot find anything there.
Looked in the IaaS API in the Network Profile section https://code.vmware.com/apis/655#/Network%20Profile/updateNetworkProfile
Also enable network tracing in Chrome to see how it's done via the UI but still cannot figure out what happens.
I can see a PUT against "https://vra01.*****.net/provisioning/uerp/resources/subnet-ranges/f3d1169e-42c3-4258-a620-7e5059ca72..."
But doesn't really help me. Any pointers?