I would use a nested if statement
the code below would display a message box if the user types /xtest hello
code:
Public Function ParseCommand(ByVal sCommand As String, ByVal sCommandArg As String, ByVal oConversationWnd As Object, ByRef sResult As String) As Boolean
If (StrComp(LCase(sCommand), "/xtest", vbTextCompare) = 0) Then
If sCommandArg = "hello" then
MsgBox "This Msgbox will be activated if user sends /xtest hello"
End if
ParseCommand = True
Exit Function
End Function