How to create a button that creates an article with a specific tag?
Sounds like the Prepopulate module does exactly what you want.

EDIT:
From the module's page:

The Prepopulate module allows fields in most forms to be pre-populated from the $_REQUEST variable.

For example, the following URL,

http://www.example.com/node/add/blog?edit[title]=this is the title


will automatically fill the Title field on a new blog post with the words "this is the title". Any field can be prepopulated this way, including taxonomy and CCK fields. You can prepopulate more than one field at a time as well. Prepopulate is excellent for creating bookmarklets. For examples on usage for all of these cases, please read the USAGE.txt file that comes with the module or you can read the online handbook page.