If the point is to get published in a journal, you need to do it their way. Their job is to take mature papers, have them vetted to make sure they are mature enough to commit to the literature, and then publish them with the stamp of their reputation. If you are unsure whether your paper merits their approval, you probably should not ask. (This is different from trying to determine publishability. If you contact an editor or two, they might give you their opinion if it does not take much time. I think Alex B. has a correct and charitable reading of the situation here.)
If the goal instead is to gauge interest in the problem, there are better ways. Probably the best is to look up conference proceedings of the area and see how much play the problem is given. Use these to get names and addresses of experts who can probably tell you how and when to approach journals, if you still want to go that route. There is less barrier to self publishing, and you can use your MathOverflow user page to talk up about yourself and your work. The user page can serve as a big business card should you wish to contact these experts.
Gerhard "Don't Need No Stinkin' Journals" Paseman, 2020.06.14.