Hi @nurzhan !
While I’m not part of the Bubble staff I think i can help you.
What you’re asking for is already a functionality present in Bubble.
(Click on the arrows below to see the images)
When you create a page that has a "type of content set":

You also have the option of choosing a field for a more readable URL, you can have a field where the user writes what he wants a path (what you call sub-domain) and there, your problem is solved

Hope it helped, if so, please leave a like and mark as solution 
Nice Bubbling !
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