Here some places which can help.

• https://community.fandom.com/Help:Template (Help)

• https://templates.fandom.com/Template:Stub (template code)

There is a pretty high chance that such a template already exists on your wiki.

Look at this location:

<yourwiki>.fandom.com/wiki/Template:Stub

You can then use it or play with its code.

There is an even higher chance that the template "MessageBox" exists, which can be used to design your own boxes:

<yourwiki>.fandom.com/wiki/Template:MessageBox