Perhaps not a direct answer, but a tweet President Trump put out on September 30th 2019 looks like an answer to your question. From what I can see, though, it's not a reply to any other tweet or question, it's merely a statement. I'll let the text speak for itself (emphasis mine):
Very simple! I was looking for Corruption and also why Germany, France and others in the European Union don’t do more for Ukraine. Why is it always the USA that does so much and puts up so much money for Ukraine and other countries? By the way, the Bidens were corrupt!
I interpret the first of the second sentence (in bold) as the reason for his request for cooperation. I think the part I emphasised refers to the Biden because the end of the tweet directly states the "the Bidens were corrupt!".