Okay, so I figured out that I was missing a little more code from my POST -method

Needed to change: 

Set data = %request.Content ==> Set data = {}.%FromJSON(%request.Content)

This got me past my previous error and to the next one, which is as follows:

"ERROR #5035: General exception Name 'READ error while reading input stream' Code '10' Data ''"

Thank you for the fast response. Sadly, after starting the production and stopping it I get the same message as I would in the management portal.

AMK>DO ##CLASS(Ens.Director).StartProduction("AMKPKG.FoundationProduction")
22:33:24.884:Ens.Director: Production 'AMKPKG.FoundationProduction' starting...
22:33:25.629:Ens.Director: Production 'AMKPKG.FoundationProduction' started.
AMK>DO ##CLASS(Ens.Director).StopProduction()
22:33:32.486:Ens.Director: StopProduction initiated.
22:33:32.513:Ens.Director: Queued messages are being saved in the suspended production.
22:33:32.513:Ens.Director: Production 'AMKPKG.FoundationProduction' suspended.

Why does it keep getting suspended and not stopping?