I think your approach is along the right lines, but if you want to persuade people to change their behaviour you have to appeal to their values. Right now your messaging sounds like changes that YOU want them to make. I’d emphasise what the change will do for them. Maybe something like:
Hi there,
Thanks for your recent post.We’ve discovered that people are much more likely to get useful (and quick) responses if they format their code correctly. I’d suggest using the </> button above the input field to format your code (select the text first, then click the button). That makes it easier for people to read and respond.