You can add credentials in two ways.
From your profile, you can click the “Edit” link in the “Credentials & Highlights” section to open the credentials dialog box:
From there, you can add a credential: by clicking “Add a credential” at the top of the dialog box:
On an individual answer, you can click “Edit Credential” in the "..." menu to open the same dialog box:
When you add a credential via an individual answer, that credential will automatically be chosen to appear on that answer.