question

The message trigger does not have a reference to the item (only to the DP itself, the sender of the message and the message parameters if there are any).

So you need to write custom code that first checks whether there is an item waiting at the DP. And if this is the case, resumes it.

Getting the item reference is the tricky part, the expression to resume it once you have a reference could be copied from a different trigger where the "Resume Item" option is available.

conveyor-resume-fm.fsm

The same logic can be build using area restriction instead. Which does not require any custom code is probably more robust, since it does not depend on the timing of when exactly the previous enters/exits the second conveyor section.

conveyor-area-restriction-fm.fsm