OTM - How to add transaction refnums using rest api?

Summary:

In OTM, how can I add transaction level refnums using REST API Transmission Resource?

Content (please ensure you mask any confidential information):

Version (include the version you are using, if applicable):

25c

Code Snippet (add any code snippets that support your topic, if applicable):

In the Transmission XML, this can be done by adding the Transaction refnums to the Transaction Header of the Transmission:
<Transmission>
<TransmissionBody>
<GLogXMLElement>
<TransactionHeader>
<Refnum>
<RefnumQualifierGid>
<Gid>
<Xid>OTZ</Xid>
</Gid>
</RefnumQualifierGid>
<RefnumValue>UTC</RefnumValue>
</Refnum>
</TransactionHeader>
<Release/>
</GLogXMLElement>
</TransmissionBody>
</Transmission>