For it you must modify source code and compiling.
Take a look at documentation - the section developer guide -> there\'s you\'ve got all you need to configure on few minutes our developement environment and check-out the openkm source code.
Take a look at this class:
Main.java ( frontend ) and comment line \"startUpPopup.show();\"
( It\'s the only change you need to do ).
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OpenKM Team ( be open my friend )
http://www.openkm.com