Sorry to resurrect the old thread, found a few other answers but not one that seems to work for me just yet.
Is there a different coding needed to find just flex fares internationally?
When I use F ..-fl as an extension code this works as expected on domestic routes (ie. YYZ-YYC) but when I use for an International route (ie. YYZ-FRA) it doesn’t work.
International fares are not branded the same way. You generally just want to search by fare class, for MUHQV.
But it's exceptionally common for PY A fares to be cheaper than even the cheapest Flex, or at least within a small margin. So it's often better just booking A if you're looking for "upgradeable 100+% earning" fares.