Step 1: Sign into the user account via https://outlook.office365.com/
Step 2: Right-click on Inbox on the left pane and select Create new sub-folder.
Step 3: Enter the sub-folder name and press enter.
Step 4: Right-click on the sub-folder and select Permissions.
Step 5: Click on the Add permission (+) icon on the top-left and select or enter the username to whom you want to share the folder.
Step 6: Select a permission level and click OK.