Normally you define wifi in your os. For your wifi image for raspberry pi there is a way to do it in General Settings->WLAN - this is just a helper interface that sends the linux commands as required.
Here I assume you mean Repetier-Server and not firmware since firmware has no wifi support except if you have a wifi to serial module, but then you need to configure the module first.
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Here I assume you mean Repetier-Server and not firmware since firmware has no wifi support except if you have a wifi to serial module, but then you need to configure the module first.