insert an update in SM using REST from external application

Hi Experts,

I need your help please.

SM 9.52. We are doing integration between 2 SM Applications. (we are only doing the integration on the interaction level)

I am able to create a record in the 2nd application.
For the Updates, it's working on all the fields except adding an activity update (the $update.action is not being updated).

For example if we modify the subcategory from the first SM application, it is being modified in the 2nd SM Application. Only the updates that we write are not being transferred.

Has anyone done this before? How can we insert an update in the interaction noting that the field is a temporary variable $update.action.

Please advise.