External sharing allowed to only designated domains

To achieve external sharing for specific domains you can follow the below steps:

1. Go to https://admin.onedrive.com/?v=SharingSettings
2. Under Limit external sharing by domain > click on Allow or block sharing with people on selected domains.
3. Click on Add domains
4. Choose any one

Allow only these domains
OR
Block these domains

5. Enter each domain on a new line.
gmail.com
yahoo.com
hotmail.com

Please refer the attached screen shots for your reference.

Reference article:
https://techcommunity.microsoft.com/t5/OneDrive-for-Business/Announcement-New-OneDrive-Admin-Center-Preview/td-p/35918