The Create ClusterUserDefinedNetworks dialog requests the admin to select existing projects this CUDN will match. While that in some cases this will be a useful functionality for secondary CUDNs, I believe it does not fit the expected flow of primary CUDN:
1. Admin creates a network matching an arbitrary label
2. Namespaces with matching label will be created and automatically registered into this network. These namespaces may be created right away, or much later.

Point being, the namespaces may not even exist at the time of the CUDN definition.

4.18 rc.8

Always

1. Open the ClusterUserDefinedNetworks dialog

It only allows me to select existing namespaces

It should allow me to select an arbitrary label. Selection of projects may stay too.