If you need this you know what it is, most others won't need to bother. This is here to be google-able.
The action is of course
The activity name is:
For PPSSPP Gold, use the following activity name:
This should work:
adb shell am start -n org.ppsspp.ppsspp/org.ppsspp.ppsspp.PpssppActivity --es org.ppsspp.ppsspp.Args "--pause-menu-exit"
But, according to issue #18528 there still seem to be some problems.