I think what you are looking for is "IV" or "Intravenous Infusion". You may receive fluids, drugs or blood by this route. It is used for large amounts of fluid. Otherwise, for small amounts it is "Injection" or "Shot" used for drugs or vaccines.

If you are simply thirsty "I need to have/get a drink" Many people now use the word rehydrate, but this does not cover medical situations beyond lack of fluids = excessive thirst; it does not cover loos of mineral salts. Calcium etc if you are conscience of the need to receive medical fluids usually salts. "I need to go on a drip" "I need to have an intravenous drip or injection"

I don’t know the Chinese characters at all, but one way to say this is “I need to get my body rehydrated”. Or more naturally “I need to rehydrate my body.”, and a final more simple possibility is “I need to rehydrate.”This is the closest I can think of. It doesn’t focus on the fluids as much but that is implied by the rehydration.

Like a flu vaccine? "I need to go get a shot".