Question Sources are a way to provide additional context for a question by attaching a relevant link. There are two ways to add a Question Source:
When posting a question
You can add a Source link below the question title (i.e. where it says "Optional: include a link that gives context")
When editing a question
To add a Source link to an existing question, click on the 'More' menu below the question title (e.g. the "..."). Choose the "Edit Question and Source" option, then you can add a Source link below the question title.
Note: you should only use Question Sources to give context to a question. To learn more, please see What is Quora's policy on question sources?.