We are trying to create an assignment in schoology and link it to an activity in our external tool. We have added the external tool in the app center and installed it to a course. We are able to click the "add materials" drop down button and access the external tool as well. However, we are unable to locate or find any documentation about the action button that is provided in the app to link content from our LTI app. 
Linking assignments from Schoology to external tool
Respondida
