How do I add signature on iPhone 8?

You’ve got to open the “Settings” App from your Homepage (Desktop), it looks like a cog wheel. Once open scroll down and on left tap “Mail”, then scroll down on right to find “Signatures” towards bottom, tap “Signatures” and you will be able to edit (it should be in the bottom box).

Note, you do not open the Mail App, it’s accessed via the Settings App.

Hope this helps.

Go to Settings/Mail and towards the bottom you should see “Signature”, tap and in the last box you should be able to add or edit text for all or separate accounts.

Just found this article that may be of interest on the iMore site, looks like it's only just be posted.

https://www.imore.com/how-change-signatures-mail-iphone-and-ipad