Creating a shared parameter will be tricky. Have to create a definition first.
Found a thread that will help and will try my luck tomorrow.
The ExternalDefinition object can be created by a definition Group object from a shared parameters file. External parameter definition must belong to a Group which is nothing more than a collection of shared parameter definitions. The following process should be followed to add a parameter to an element: Open the shared parameters file, via the Application.OpenSharedParameterFile() method. Access an existing or create a new group, via the DefinitionFile.Groups property. Access an existing or create a new external parameter definition, via the DefinitionGroup.Definitions property. Create a new Binding object with the categories to which the parameter will be bound using an InstanceBinding or a TypeBinding object. Finally add the binding and definition to the document using the Document.ParameterBindings object.