"I want to go there."
Translation:Ik wil ernaartoe.
That's fine. "Er" often forms one word together with a preposition, as is the case with "erheen". And there are lots more, see this list: https://onzetaal.nl/taaladvies/advies/er-voorzetsel-werkwoord
I'm not native Dutch so CMIIW. Er-naar-toe can roughly be translated it as there-to-go/be. As a compound noun it implies already the action of going. As WarmFoothills mentioned, the complete sentence is "Ik wil ernaartoe gaan" but gaan is often ommited. People also say "Ik wil naar de markt", Ik wil naar school, etc.