Google Message Continuity Setup and Administration Guide - Manual (Page 15)

Change Your Settings in the Control Panel 19
4. From the Advanced tools page, enable Two-Legged OAuth:
a. In the Authentication section near the top of the screen, click Manage
OAuth domain key.
This opens the Manage OAuth key and secret for this domain page.
b. In the OAuth consumer key section, make sure the Enable this
consumer key check box is selected.
You will later need the OAuth consumer key when you configure the
Continuity Sync Server and when you configure the Continuity Settings in
the Google Apps control panel.
c. If the OAuth consumer secret is not yet displayed, click the button to
generate the consumer secret.
Important: If the OAuth consumer secret is already displayed, do not
click the Regenerate OAuth consumer secret button. Instead, keep the
existing key.
d. Under Two-legged OAuth access control, click the Allow access to all
APIs check box.
e. Click Save changes.