Here are some possible reasons why the Host Name field may be greyed out in Microsoft Office:

• The option to add trusted locations using hostnames may be disabled by your organization's IT policies. In this case, you will need to contact your IT administrator to check if this option can be enabled.
• You may not have sufficient permissions to add trusted locations using hostnames. In this case, you will need to contact your IT administrator or a user with administrative privileges to grant you the necessary permissions.
• There may be a technical issue with your Microsoft Office installation or settings. In this case, you can try repairing your Office installation or resetting the Trust Center settings to their default values.

To reset the Trust Center settings in Microsoft Office, follow these steps:

1. Open any Office application (such as Word, Excel, or PowerPoint).
2. Click on the File menu and select Options.
3. In the Options dialog box, select Trust Center from the left-hand menu.
4. Click on the Trust Center Settings button.
5. In the Trust Center dialog box, click on the Reset all Trusted Locations to the default settings option.
6. Click OK to save the changes and close the Trust Center dialog box.

After resetting the Trust Center settings, you should be able to add trusted locations using hostnames if the option is available.

Note- Make sure the following GPO settings are applied-