So far so good, you added the micrflow-button, called the microflow, opened the Select form and show the options. Now upon selecting an option from the list, you have to call the Change-activity for the parent-object and Add the selected child-object to the parent-object. Therby having Type set to ‘Add’. after this you add the ‘Close page’ activity and you should see the selected option being added to your parent-object.
Here is an example (without closepage):