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 Sharing policy list, select the sharing policy you want to apply to this mailbox.
Step 6: Click save to apply the sharing policy.