The Mail tip users get that someone is out of the office is triggered when the Out of Office is enabled.
If you as an admin wanted to simulate this, you would set the mail tip on that mailbox to say they were out of the office and then the user would not actually enable their Out of Office reply
https://video2.skills-academy.com/en-us/powershell/module/exchange/set-mailbox?view=exchange-ps
-MailTip
The MailTip parameter specifies the custom MailTip text for this recipient. The MailTip is shown to senders when they start drafting an email message to this recipient. If the value contains spaces, enclose the value in quotation marks (").
When you add a MailTip to a recipient, two things happen:
HTML tags are automatically added to the text. For example, if you enter the text: "This mailbox is not monitored", the MailTip automatically becomes: <html><body>This mailbox is not monitored</body></html>. Additional HTML tags aren't supported, and the length of the MailTip can't exceed 175 displayed characters.
The text is automatically added to the MailTipTranslations property of the recipient as the default value: default:<MailTip text>. If you modify the MailTip text, the default value is automatically updated in the MailTipTranslations property, and vice-versa.