Hi!
You can do this by going to the top right of the forum tab; there you will find a button with the format [ICON / PROFILE PHOTO] [USERNAME].
By clicking it, you will find various sections of your account. Among these you will find the option "Signature".
When you click again you will fin a text field in which you can insert from a text to moving images (GIFS).
At the top of the field you will find some buttons with which you can customize and give your signature a unique style.
Finally, having finished your signature, we click the "Save" button to not lose the changes.
Now you will accompany your post with a signature that will decorate them every time you post messages or generate topics!
I hope this little tutorial has been helpful to you!