Go to Pure (https://pure.knaw.nl/)
Select the research output that you want to add Creative Commons License (CC) to.
Note: only if the electronic version of a record (File, DOI, or Link ) is available it is possible to add CC to the record. Otherwise, you need to first add the electronic version to the record and then it is possible to select CC.
Scroll down and click Edit:
Select the proper license to Document (What is Creative Commons License?):
The Creative Commons License are often embedded in the HTML/PDF view of an article (in the beginning or at the end of an article) or available from publisher’s website.
Click Create/Update.