Usually you can access the ingame command input for the server through double-clicking the tilde (~) key. A command line awaiting your input should appear at the bottom of your screen.

Update: As of version 171 the part below is no longer neccessary. You can now simply change the Keybinding in the Options-Menu.

Somethimes though, this doesn’t work (the command input simply doesn’t appear). In this case, the following tip might be helpful. Go to the file

YourInstallationDirectory\ARK\ShooterGame\Config\DefaultInput.ini

At the very end of this file you will find the following lines.

ConsoleKey=None
+ConsoleKeys=Tilde
;+ConsoleKeys=Tab

Replace them with this:

ConsoleKey=None
+ConsoleKeys=Tilde
+ConsoleKeys=Tab

(or, alternatively, in case the above fails you, with this:)

ConsoleKey=Tab
+ConsoleKeys=Tilde
;+ConsoleKeys=Tab

Save the file and restart the game. You should now be able to access the Server Command Input via the TAB key.