Hi @Udesh Tamang , Internal reply message content is mandatory when setting it up via EAC. If you only want to enable it for external users, you can connect to Exchange Online PowerShell and run command similar like below:
Set-MailboxAutoReplyConfiguration -Identity shared1 -AutoReplyState enabled -InternalMessage "" -ExternalMessage "External auto-reply message." -ExternalAudience All
Another method is that, assuming you have full access to the shared mailbox, you can open the shared mailbox via OWA > Open another mailbox,
Enter the identity of the shared mailbox so that you can access it. Then you can set the automatic reply for the shared mailbox there. Based on my test, it's okay to leave the internal configuration blank when saving the configuration:
Hope the above can be helpful.
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