In the comments some concerns were raised that this idea (or some variation thereof) could lead to some users losing the question they typed simply by clicking incorrect field.
@MattF., that seems like a great idea. The only concern I'd raise is: would the 'wrong' answer automatically trigger rejection of the post, or just a warning that it might not be appropriate? If the latter, then people might just ignore it as they would ToS. If the former, then good questions could be lost to people giving overly honest answers. -- LSpice
So I'd like to clarify that the proposal is to show How to ask page (or something similar) before the user starts typing their first question.
To check how this work on other sites, I created a completely new account on Mathematics. Immediately after creating an account, the invitation to view the tour was shown in my inbox. (However, this is easy to ignore. Still it is interesting that this was not shown when I created MathOverflow account - but perhaps a user gets this into inbox only for the first account they create...?)
Then I clicked on "Ask question" button and I was shown How to ask page:
At the bottom, the checkbox saying "thanks, I will keep these tips in mind when asking" was shown.
If I clicked on proceed, I was directed to the same page again. Only after checking the checkbox, clicking on proceed took me to the page where I enter the question.
When I tried this with an entirely new account on MathOverflow I am taken directly to https://mathoverflow.net/questions/ask - the system does not force me to read anything before that. (Of course, the same thing happens when I try to ask a question on MathOverflow while I am not logged in at all.)