Reference:/zh-CN/documents/outlook/1114-outlook-create-template.html

Click on the email you want to use as a template, File -> Save as OutLook template ( .oft format file), saved in the local disk folder.

File -> Options -> Customize Ribbon -> Check Development Tools, as shown below

Return to the main menu, Development Tools -> Select Form

Select the search scope as "User templates in the file system", click Browse to find the directory where the template is located, and open the desired template in that directory.

Note that a signature will be added when sending an email using a template, so you can delete the signature in the template email to avoid having two signatures.