That is actually a good question and a difficult one to answer, too. Because it depends on every single verb.
For instance, 'möchten' can have 2 structures: either 'ich möchte es' or 'ich möchte, dass ...'. So, 'ich möchte es, dass ...' is wrong.
For 'helfen', the situation is different, the prepositional pronoun is optional, so both sentences are correct.
But for other verbs, let's say 'es geht um' it is mandatory: Es geht darum zu verstehen, wie diese Verben funktionieren.
How to know which one is optional and which one is not? There is no rule. Basically, you have to know for every single verb, because very single verb has its own structure.