Hi, I would like to create a VNet subnet in a separate resource group from the resource group in which the VNet was created. I only see the option to specify the VNet name, but then it looks in the same resource group as the subnet and does not find the VNet. I think I need to specify the VNet ID instead, but I cant see how to do that

Subnets aren't full resources, they're a part of the vnet.

Okay. I just thought I read that there was a way to have a subnet under a separate resource group. Perhaps not though.

No Subnets are attached to V-Nets. Subnets themselves are not resources.