I can't find a direct answer.

Is the rating of a power focus added to drain resistance tests?



I can read this section as it dose or doesn't.

If it dose how about the drain from Drakes abilitys?

It doesn’t help with drain unless your drain test is based on your Magic rating.

The focus works by adding its rating to your Magic rating. It doesn’t affect any other attributes other than Magic

Exactly. The key part is "any other test where Magic is involved".

However, since they change your effective Magic rating (different than SR4, where they only added dice to tests), they do change Drain in one important way. They allow you to get more hits before Drain does physical, rather than stun, damage.

Example: Bob the mage casts fireball at Force: 4, getting 4 successes after the limit is applied. He has a Magic of 3, so normally he would resist physical damage from Drain, since his successes are over his Magic rating. But he has a Force: 2 power focus active, raising his effective Magic up to 5, so he only resists stun damage from Drain.

How would you gain the ability to use Magic as a drain stat? I had a quick look at the magicians traditions listed in chummer (I don't have street grimoire yet) and there doesn't seem to be an option that allows that. That sounds dangerously close to encouraging people to be single attribute dominant.

You can't, so they don't. But it does help with just about anything a mage does except for assensing and drain, so it's quite good. Even adepts can find (limited) use, for things like attribute boost, although it's quite expensive just for that. But mystic adepts with both have a very good use of this thing.

In Forbidden Arcana is a tradition taught by dragons to their drakes, simply called the Drake tradition. Among other disgusting benefits, your drain attributes are Willpower & Magic. The catch, you must be a dragonborndrake and beholden to one of the Greats.

That would mean that yes, if you are a drake who uses the dragon tradition then a power focus would help with drain.
If any ability says that drain is based on the Magic Rating then a power focus would help there too.