Can you put a signature at the end of a message ?

If you are referring to an SMS or iMessage, there currently is nothing in place to automatically place a signature at the end of the message. Many people have used the Text Replacement feature in Settings that allows for creating text strings that can be automatically entered when you use a shortcut. Go to Settings>General>Keyboard>Text Replacement, then create a replacement by clicking on the ➕.

Then in the Phrase section create your signature, and in the shortcut section a two or 3 letter shortcut that would enter that phrase, i.e. Phrase: YourSignature; Shortcut: abc. Then when you type abc at the end of your message, YourSignature would automatically be entered when you press the space or Send button.