I have the following text on some Sanuki Udon I brought home with me ;) This is the part for ざるうどん:
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．