Locating Action URL

[ExodusRP]

I've been trying for some time now to locate the action url on web.de but I'm unlucky so far, I keep getting error 430 but I know for fact I just have the wrong Action URL, I already debugged the site and the "actual login URL" is

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Upon placing this as action URL I get error 400, bad request. Anyone got any ideas?

Thanks in advance.