Manually update the app.config xml file (and access the properties via ConfigurationManager.AppSettings["key1"]; )
<appSettings>
<add key="key1" value="value1"/>
<add key="key2" value="value2"/>
</appSettings>
Or you can Select your Project, then Properties and Settings:
( access the properties in the code with Properties.Settings.Default.TestEnvironment)
<applicationSettings>
<Projectname.Properties.Settings>
<setting name="TestEnvironment" serializeAs="String">
<value>True</value>
</setting>
</Projectname.Properties.Settings>
</applicationSettings>
References: