thanks
The combat ship must be set to defend the freighter. Add the freighter's position number to its defend list. Or even the whole affiliation, which has a similar effect (but not exactly the same!).
The combat ship must also have its Combat Options changed to Never flee and On Patrol must be set to On.
Alert Status must also be set to On (default setting).
Number of targets, ammo loadout, target preferences and retarget options are not strictly relevant.
The above steps will guarantee that the combat ship will defend the freighter if it comes under attack. Obviously it would make sense to pair them up in a fleet (political orders) so that they are always moved together, and stay together. If you move them separately then one may come under attack and the other may happily move on because it wasn't attacked, only to run into another enemy elsewhere later on its turn.
The second layer is defense within combat: Screening.
You can set your combat ship to Screen the freigher. It will then try to break enemy lock on the freighter by drawing fire to itself. I don't have experience with this so cannot say if it works well. The ships should be vaguely similar in size for this to work, or so I am told. Perhaps someone else can elaborate.
Hope this helps.