If you struggle entering your key directly into the game, you can do it through the Windows registry base.
[list type=decimal]
[*]Do ALT + R, a box 'Execute' or something of the sort should open
[*]Input 'regedit.exe' and press [ENTER]
[*]Go to HKEY_CURRENT_USER/Software/MountAndBladeWarbandKeys
[*]Change the content of "serial_key_nw" for you Serial Key
[*]If it does not exist: Right Click → New → String Value → Name it "serial_key_nw" → Double click and enter your CD Key.
[*]Try to connect to a server
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