Step 1: Sign into Office 365 admin portal via https://portal.office.com
Step 2: Click on Admin from the left pane and navigate to Exchange admin center.
Step 3: Click on the Recipients and switch to Shared tab.
Step 4: Click on the shared mailbox you would like to grant access and click on Edit icon on the top-left.
Step 5: Switch to Mailbox Delegation and scroll down to Send As section.
Step 6: Click on the Add icon and select an user from the list and then click Save.