See explanation below
Explanation:
From the Microsoft 365 admin center dashboard, go to Admin > Exchange.
Go to Organization > Sharing.
Under Organization Sharing, click New +.
In new organization relationship, in the Relationship name box, type a friendly name for the organization relationship.
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. If you need to add more than one domain, you can do it after you create the organization relationship by editing it.
Select the Enable calendar free/busy information sharing check box to turn on calendar sharing with the domains you listed. Set the sharing level for calendar free/busy information and set which users can share calendar free/busy information.
To set the free/busy access level, select one of the following values:
- Calendar free/busy information with time only
- Calendar free/busy with time, subject, and location
To set which users will share calendar free/busy information, select one of the following values:
- Everyone in your organization
- A specified security group
Click Browse to pick the security group from a list, then click OK.
Click Save to create the organization relationship.
Reference:
https://docs.microsoft.com/en-us/exchange/sharing/organization-relationships/create-an-organization-relationship