SSIS Script task and web service, how to change URL when dynamic

I have a script task that accesses a web service. I cannot use the web service task as I need to set header information which is not in my sample below.  In the code window i set a web reference to my web service and the URL is set to dynamic. I need to change this URL based on environment. The URL appears to be stored in the app.config file. My question is how do I change this value on the fly? I can pass in variables to the script itself but do not see a way to modify the app.config file. I have seen other examples but none meet my requirements.

Here is my code and the configuration file example. Is there a better way that I am not aware of?

This is a simplified example

TestWS wsCall = new TestWS();

int returnValue = wsCall.DoSomething();

App.Config File

I am trying to change the Value of the URL at runtime

 <applicationSettings>
        <ST_ebae43bfbdb74b829486e1f379544e4f.csproj.Properties.Settings>
            <setting name="ST_ebae43bfbdb74b829486e1f379544e4f_csproj_TestWS"
                serializeAs="String">
                <value>http://localhost:1485/TestWS.asmx</value>
            </setting>
        </ST_ebae43bfbdb74b829486e1f379544e4f.csproj.Properties.Settings>
    </applicationSettings>