To create a new API key, head over to your Profile + Settings by clicking the person icon on the top right.
Head over to the OnDMARC tab, and towards the bottom of the page, you will see the API Keys section that will allow you to see your current keys and add new ones.
To add a new key, click the ADD button which will display the below screen:
Once the roles have been assigned for this key you will be shown the API key.
Note: This key will only be shown once. You cannot view the key again after this point.