It is possible but can be challenging. If UA codeshares on the route, you flat-out will not be able to book it online, because United.com will give you the codeshare and there is no F on the codeshare. If not, sometimes you might see it online, but UA hasn't been showing as many non-UA flights as they once did.

Therefore, your only hope is to call and find a UA agent who will be wiling to do it. It may be easier if you include a UA code on your itinerary somewhere, on some non-F connecting flight; agents are allowed to issue the tickets either way but you may get pushback if there are no United segments, as not all agents understand the rule.