There is no validation of the NAD name when creating a NIC.
The bug originally submitted for the UI however the invalid name was accepted also by the backend.

4.20

always

1.in the select type invalid NAD name i.e. foo/not.valid
2.use that name to create an option in the dropdown
3.use that option for the created network

the network is created but the VM cannot be started as it's not possible to create such NAD - failing condition is
message: 'failed to render launch manifest: failed to locate network attachment definition foo/not.valid.name'      
reason: FailedCreate      
status: 'False'      
type: Synchronized

invalid name is blocked at creation time