A somewhat more complete solution that sets also the tags and category would look like this:
<$list filter="[all[current]tag[$:/tags/Discourse]]">
<$wikify name=url text="""http://talk.tiddlywiki.org/new-topic?title={{{ [{!!title}encodeuricomponent[]] }}}&body={{{ [{!!text}encodeuricomponent[]] }}}{{{ [all[current]get[category]has[subcategory]encodeuricomponent[]] :reduce[<..currentTiddler>get[subcategory]encodeuricomponent[]addprefix[/]addprefix<currentTiddler>addprefix[&category=]] :else[all[current]get[category]encodeuricomponent[]addprefix[&category=]] }}}{{{ [all[current]tags[]] -[[$:/tags/Discourse]] +[encodeuricomponent[]] +[split[ ]join[,]] +[addprefix[&tags=]] }}}""">
<a href=<<url>> target="_blank" class="tc-tiddlylink">{{$:/language/PostToDiscourse/Hint}}</a>
</$wikify>
</$list>
Create a tiddler tagged $:/tags/ViewTemplate and place this code inside
Create the tiddler $:/language/PostToDiscourse/Hint and write something like “Post to Discourse” in its text field
Then all tiddlers tagged $:/tags/Discourse will have a “Post to Discourse” link at the bottom
All additional tags to the tag $:/tags/Discourse will show up as tags in the Discourse dialogue
The field “category” of the tiddler is used to set the category, the field “subcategory” is used to set the subcategory