Pass argument to sbt task

I have the following sbt task that I'd like to pass an argument to:

import sbt.complete._
import complete.DefaultParsers._

val stageEnv: Parser[String] = " staging" 
val prodEnv : Parser[String] = " production"
val combinedParser: Parser[String] = stageEnv | prodEnv

slick := slickCodeGenTask.value

lazy val slick = inputKey[InputTask[Seq[File]]]("gen-tables")

lazy val slickCodeGenTask = Def.inputTask {
  ...
  s.log.info(combinedParser.parsed.trim)
  Seq(file(fname))
}

However, when I run it and pass an arg like 'sbt slick prodEnv' I get the following error:

[error] Not a valid command: prodEnv
[error] Not a valid project ID: prodEnv
[error] Expected ':' (if selecting a configuration)
[error] Not a valid key: prodEnv (similar: products)
[error] prodEnv
[error]