Custom field referencing custom field in API

Hi,

I am fairly new to API's, but I am learning quickly. I am trying to figure out if I can write a custom field in the API to reference another custom field.

I have set the subject with the API using the {\"subject\": \"RETR-OGI-123456\"} so what I am trying to do now is to tell the custom "SAP RMA Number" field to reference what is in the subject and use that as the text value. 

Just as a Macro allows you to input the placeholders for inserting custom field contents into the comment/description with something like this 

{{ticket.ticket_field_14541708058898}} (this takes my custom field "Reported Issue" and inserts it into the comment/description section of the ticket,

 I am trying to do the same with inserting the contents of the ticket subject into the custom field "SAP RMA Number". I know that the below API example will not work, but it's just an example as to what I am trying to accomplish.

Hopefully this makes sense. If anyone has any experience with this and has any suggestions, I would greatly appreciate it. Thank you in advance.