I have basically won 200k on a casino online website called royalbet.top and thier asking me to to pay 2304$ for thier tax fee is that legit?

on the face of it NO.  if you can contact them ask why can't they take the 2304$ out of your winnings. 

This doesn't pass what accountants refer to as the smell test. However, we have no way be sure.

think of it this way if you get a w-2 they take the deductions/taxes out and pay you the net. they don't ask you to front the deductions and then pay you the gross 

You may want to ask what exactly their "tax fee" is for.

Legally, they just need to give you W-2Gs (or depending on how it is structured, 1099-MISCs) that report your large winnings.

I don't know what it is for, but it is hypothetically possible that they are asking you to pay that fee to get an itemized list that would enable you to report the "session method".  In SOME cases, that can significantly lower the tax that you owe (but in other cases, it could increase your tax versus only reporting your W-2Gs).