How to apply a sharing 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 Sharing policy list, select the sharing policy you want to apply to this mailbox.

Step 6: Click save to apply the sharing policy.