In Taverna 2.x, you can set a constant value by right-clicking on the appropriate input port and selecting Constant value. Alternatively, you can drag in a String Constant from the Service Templates section of the Service Panel and connect it to the appropriate input port of your service.
In Taverna 1.7.x, you can set a default value for an input port of a processor by right-clicking the port node in the Advanced Model Explorer tree and selecting Set default value.
