Add a Rule with the following:
- a "Click" event (from the Core extension), with the appropriate selector for your "Download" button
- (optional) any conditions that are required
- a "Set Variables" action (from the Adobe Analytics extension) to set the required props/eVars/events
If your "Download" button links to a downloadable file, e.g. "something.pdf", then Adobe Analytics will automatically track the Download Link, and pass the variables that you had set in the above Rule. You don't have to add a "Send beacon" action.