@Simm  -  Are you trying to set the variables value of the request which you are creating it through script?

If yes then please you have to create a record on "sc_item_option" table then tag this record with RITM  you need to create a record on "sc_item_option" table.

Or better use Cart API to submit the record if the business is fine with new REQ for each RITM and you can link initial RITM as parent to the new ones.

Regards,
Abhit.