Slitherine

I have a finished scenario I would like to post. What is the best way to post the URL download link? Link to a Dropbox folder that others can access? Will that work?

Yes.
I have used dropbox for backups for years and distributing stuff here.
There are surely (free) alternatives although these are usually cluttered with commercials.

Follow the instructions in the first post of the thread where you originally posted (I moved your post here), particularly the part about "You must have a discussion thread in the Scenario Design forum in which you will maintain the link to, and version number for, your scenario or campaign."

When you successfully post in that thread concerning your custom work, using the template provided, the post is then moved to either the Custom Single Player Scenarios, Custom Multiplayer Scenarios, or Custom Campaign thread in this forum, depending on what it is. The "Announce" thread is not for general discussions.