Here's a quick guide about how you can add & make your customised signature to Yahoo Mail in Outlook App easily. Just follow the given steps carefully:
1) Open your Outlook Mail & Go to the Settings.
2) Now click on the All Settings Option.
3) Then, scroll down the left options till you get the Layout Category.
4) Under the Layout category, there will be an option of "Email Signature"
5) Now, you can see a text bar on the screen, where you can make and add your Signature in Yahoo Mail easily.

Source: https://emailspedia.com/how-to-add-signature-in-yahoo-mail/