You can set a personal signature that is automatically added in a message by adding a text to the plain text field message signature. This field is only suitable for text, it is not possible to add HTML code or images.
Setting a message signature
After logging in, click on your username at the top right. Then add a text in the plain text field message signature. Finally, click on the Save changes button at the bottom.
Send a message
If you create a new message on the right side of the timeline, the message signature will automatically appear with every message you have created.