在创建步骤时将页面添加到某个命名空间

I want to add the page in a certain namespace on step creation. How it must look like: a dropdown with list of namespaces (required for checking) and after pressing "post" button page will be placed in selected namespace.

What i do: - edit wikipage.php (function insertOn() ) - editeditpage.php on string 2242

And... I have error on posting "The Title object did not provide an article ID. Perhaps the page doesn't exist?"

Anybody do it?

There are two different things in your request:

• a dropdown to select categories during page creation;
• enforcement of category selection.

So I think you should address the two things separately:

• install VisualEditor to provide a GUI for category addition (perhaps configure it to make the category selector more visible, but that's offtopic);
• install AbuseFilter and add a filter which disallows creation of a page without a category; in the error message put some friendly text to let users know they must use said feature.