日本へ___です。
Q. Which form expresses 'I want to go' to Japan?
Translation
I want to go to Japan.
Explanation
〜たい attaches to the masu-stem: 行きます→行き+たい=行きたい. 行くたい wrongly attaches たい to the dictionary form, and 行きたく is the adverbial form which cannot stand before です here.