To add a keyword to the protocol of which you are the creator, you need to edit the protocol in the Protocols repository.
First, go to the Protocols repository (you can access it in the left bar of the dashboard). Select the protocol to which you want to add keywords and click the Edit button. 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 the green Update button.
For easier understanding see the GIF below.