There are two ways this can be achieved.
Workround 1
It should be possible to add a domain to the Find Domains section in MNE and this in turn should then populate a proxy address to the collection when you run the Export Notes directory again.
Workround 2
Manually edit the collection's TSV file and import it back in with the proxy address required.
Note: Make sure that the file encoding remains UTF-8 when saving the TSV