The interview code may be at the bottom of the interview email invitation. The position of the interview code in the email may vary depending on the recruiting company.
If you are using a mobile device for the interview, you will need to download the HireVue app. (When you search the App Store or Google Play Store, it's called "HireVue for Candidates"). After downloading the HireVue app, go back to the invitation email and click on the option to start the interview. The interview code will populate the HireVue app.