Categories can’t have default tags. That wouldn’t make sense to do either, because tags and categories are different dimensions of organization for your topics.
You can however create a link or a bookmark which will create a topic that has tags pre-populated, this is the syntax:
http://discourse.example.com/new-topic?title=topic%20title&body=topic%20body&category=category/subcategory&tags=taghere
You don’t have to specify the title or body using the link, if you just wanted to pre-populate category or tag you would just use those parts. This bit of the URL is mandatory:
http://discourse.example.com/new-topic?
And each field is separated by a &. If you want to use a space in any field then use %20 which will be automatically converted.