For details see my blog post http://foldingair.blogspot.co.uk/2015/11/splunk-alerts-to-slack-using-powershell.html

SplunkSlackAlert.bat

@echo off
powershell "C:\Program` Files\Splunk\bin\scripts\SplunkSlackAlert.ps1 -ScriptName '%SPLUNK_ARG_0%' -NEvents '%SPLUNK_ARG_1%' -TriggerReason '%SPLUNK_ARG_5%' -BrowserUrl '%SPLUNK_ARG_6%' -ReportName '%SPLUNK_ARG_4%'"

SplunkSlackAlert.ps1

param (
    [string]$ScriptName = "No script specified",
    [string]$NEvents = 0,
    [string]$TriggerReason = "No reason specified",
    [string]$BrowserUrl = "https://localhost:8000/",
    [string]$ReportName = "No name of report specified"
 )

$body = @{
    text = "Test for a parameterized script `"$ScriptName`" `r`n This script retuned $NEvents and was triggered because $TriggerReason `r`n The Url to Splunk is $BrowserUrl `r`n The Report Name is $ReportName"
}

#Invoke-RestMethod -Uri https://hooks.slack.com/services/AAAAAAAAA/BBBBBBBBB/CCCCCCCCC -Method Post -Body (ConvertTo-Json $body)