OK, I skimmed the manual. The marker RF is mixed in with the swept RF output so no post-injection.
You'll need to set the marker frequency so that it falls within the range being swept and passing through the IF stage under test. You should see it as a little blip on the scope.
Try building the broadband detector they show on page 5 - it's a good way to make sure your 368 is working properly.
You might also find my videos on using a Hickok 615 sweep marker generator useful. I show what the blip should look like.
part 1
part 2