How to apply a retention policy to a single mailboxes from EAC (Exchange admin center)

Step 1: Sign into Office admin portal via http://portal.office.com and open Exchange admin center

Step 2: Navigate to Recipients > Mailboxes.

Step 3: In the list view, select the mailbox to which you want to apply the retention policy, and then click Edit icon.

Step 4: In User Mailbox, click Mailbox features.

Step 5: In the Retention policy list, select the policy you want to apply to the mailbox, and then click Save.