How to pick answers category before posting the question?

There is no choice to Choose the Category before Submitting now.

On Mobile switch to Desktop.

The Categories are Chosen based on key words and it is often wrong but not any more so than when the asker could Choose the Category before posting.

Once you submit your Question there is Edit Category above your Question, click that.

Click All to see all Categories click the one you want and then find the Subcategory click that and then Submit.

The down side is any level 7 users can move any Question to any Category so if you see your Question is moved you can move it back.

If you are on the newest format with Round Avatars click Edit under the question and then Edit Category and do the same as above..

You would have to be on the mobile version of the site; desktop and computer version of the site has removed that ability and replaced it with the ability of being able to endlessly choose the category after the question has been posted.

You can't because the category can only be chosen after posting

That option is no longer available. The best you can do is include certain key words that will effect the automatic placement of your question. For example:

"Mathematics category question: What's 10X10?"

try using key words when typing your question

you have to do it after you ask the question.