How do I automatically add signatures in Outlook app?

The client side plugin.

You can either create a server side rule that automatically adds the signature, or you can deploy the outlook plugin and set it to automatically add a signature. https://www.codetwo.com/userguide/email-signatures-for-office-365/create-rule.htm

The easiest way is to use the server-side signature. Users don't need to do anything. The signature is added automatically based on the settings in Code Two. The user wouldn't see a signature, but they also don't need to worry as the system adds it per the rules created. There are client-side signatures that can be added with the code two client-side signature rules, but I would argue that is completely unnecessary in most cases.