By mentioning Crowdstrike in the phone call, the U.S. president was trying to get Russia off the hook for the 2016 email hacks.
Trump and his supporters have since pushed a conspiracy theory that the Obama-era FBI investigation into the DNC hack was not thorough and that Crowdstrike somehow falsified information to harm Trump. There is no evidence for this. The intelligence community concluded that Russia hacked the DNC. Even the National Republican Congressional Committee later hired Crowdstrike to provide cybersecurity services before the 2018 election.
In broaching this “favor,” Trump is essentially asking the president of a country under attack by Russia to help him undermine the findings of the U.S. intelligence community. He is also asking Zelensky to help conceal the activities of Russian GRU agents who did the 2016 hack and have since attacked Ukraine.