Step 1: Sign into Office admin portal via http://portal.office.com
Step 2: From the Microsoft 365 admin center dashboard, go to Admin > Exchange.
Step 3: Go to organization > sharing.
Step 4: Under Organization Sharing, click New Add Icon.
Step 5: In new organization relationship, in the Relationship name box, type a friendly name for the organization relationship.
Step 6: In the Domains to share with box, type the domain for the external Microsoft 365, Office 365, or Exchange on-premises organization you want to let see your calendars.
Step 7: Select the Enable calendar free/busy information sharing check box to turn on calendar sharing with the domains you listed.
Step 8: Set the sharing level for calendar free/busy information and set which users can share calendar free/busy information.
Step 9: Click Save.