You use winset() to change the input control's command parameter. You add an exclamation mark (!) before the command you're setting for it to stay as the default.
There is also the deprecated command_text var to do this, but you shouldn't use this as it's older and less flexible.

You can read about winset() and other procs (and operators, vars and more) in the DM Reference for more info, and you can also read about interface controls and their parameters in the Skin Reference. These are both also available from Dream Maker itself.