URI this is only relevant if they want to point to a term that already exists. In many instances this will be something that the admin is creating custom for a space and therefore we should auto-generate the URI and show it in the list up above. So, I think this needs to be an optional field and say in the pop-up help that it's to point to an already existing metadata term e.g. in Dublin Core.

Acceptance Criteria (From Dharma's Comment):
1. the URI field is not required.
2. Help text appears with the URI field that reads "If the term already exists, add the URI where the term is defined. If this term is custom to the Project Space, SEAD will generate a URI for the new term which will appear in the listing above."
3. If the user doesn't enter a URI, SEAD generates a URI for them and it appears in the list for the space.