To add a keyword to the protocol you created, you need to edit the protocol in the Protocols repository.
First, go to the Protocols repository (you can access it in the left menu). Select the protocol to which you want to add keywords and click Edit. Click on the keywords icon and write the keywords into the blank field, pressing enter on your keyboard after adding each keyword. When you are finished with writing the keywords, click Update.
For easier understanding see the GIF below.