In the WSDL view, right-click the Extensibility Elements node and choose Add -> Property.
A property node appears under the Extensibility Elements node.
If the Properties window is not visible, choose Window -> Properties.
Select the property node.
Set the name property.
You can set the ElementOrType property to a built-in simple type, such as string or date. If you added an inline schema or imported a schema, then you can select a type from the schema.