I have the following text on some Sanuki Udon I brought home with me ;) This is the part for ざるうどん:
水洗いした麺をざるに盛り、あらかじめ用意したつけ用つゆに、生姜、ねぎ、ごまなどお好みの薬味を添えてお召し上がりください。
So, つゆ is the sauce or soup, あらかじめ用意した means "prepared beforehand". So what is the task of つけ用? If I must guess I would say "prepared". Unfortunately Weblio doesn't have this in its data base. But then, why not simply leave it with "あらかじめ用意したつゆに..."?
And is my reading correct? I would say つけよう．
Many thanks!
「かけ」 vs. 「つけ」
Those are two of the more common serving styles of udon.
「かけ」 comes in one (large) bowl with both the broth and noodles in it.
With 「つけ」, the noodles and broth are served separately for you to do your "dipping and dunking". You get the noodles in a dish or shallow bamboo basket and the broth in a small bowl/cup. That bamboo basket is called 「ざる」; therefore, 「つけうどん」 is generally synonymous to 「ざるうどん」.
Point is the thickness of the broth is different for the two styles. The つけ broth is much thicker and saltier than the かけ, which is why you can "drink" the かけ broth.
Thus, by specifically asking you to prepare 「つけ用{よう}つゆ」, I think the manufacturer wants to prevent you from preparing the "wrong" broth by mistake, which is the thinner 「かけ用つゆ」.
(What we often do is to buy a bottle of concentrated broth and add water at different ratios depending on how we want to eat the noodles.)
I think your version of saying あらかじめ用意したつゆに sounds better. The writer simply wanted to emphasize the source is for dipping, not just a regular broth. Japanese language uses repetitive modifiers already stated as an adjective.
