Sorry, it’s the aXML chunk, not the iXML. I misremembered.
This was from the EBU document defining this last year
https://tech.ebu.ch/docs/tech/tech3352.pdf (section 5)
The only way I know to do this in BWFMetaEdit is to copy and paste the example code from the EBU document into the aXML chunk, replacing the example ISRC that’s in the code with your real ISRC. This is the code to copy and paste:
<ebucore:ebuCoreMain xmlns:dc=" DCMI: DCMI Metadata Terms" xmlns:ebucore=“urn:ebu:metadata-schema:ebuCore_2012”>
ebucore:coreMetadata
<ebucore:identifier typeLabel=“GUID” typeDefinition=“Globally Unique Identifier” formatLabel=“ISRC” formatDefinition=“International Standard Recording Code” formatLink=“http://www.ebu.ch/metadata/cs/ebu_IdentifierTypeCodeCS.xml#3.7”>
dc:identifierISRC:NOX001212345</dc:identifier>
</ebucore:identifier>