Hello,
The messages you was looking for is called 'AssociateEntities' and is by default 'IsCustomProcessingStepAllowed = false'
You can find the id for the message in 'SdkMessageBase'. With that id you can set 'IsCustomProcessingStepAllowed' to true.
Next you have to make the following changes to the Plugin registration tool:
When you register the plugin you have following restrictions:
Best Regards, Lassaad