Need help with implementing Drop Down Menu in Post message

There may be ways of doing this through Studio, but I can't say that I would necessarily recommend it as the approach doesn't use established customization contract points and would be prone to breaking with upgrade. The best way of doing this would require customization of the message editor form and the addition of custom message metadata, both of which would need to be done by Lithium Professional Services. Storing the values in metadata would have other benefits as well such as being able to search/filter by specific metadata values via the REST API. If you'd like to get this scoped out by the Professional Services team, please file a case via the customer case portal.

If you do want to try to tackle this yourself, the high-level approach might look something like this:

1. Add a custom component to the post page that displays the drop down menus. You're likely going to run into issues getting the exact placement as shown in your image as the message body editor is a single block.
2. Add some JavaScript to capture the form submission event and prevent the default action.
3. Grab the content from the custom drop down menus you added, format it, and append to the message body value.
4. Submit the form with the updated value

Again, I would advise against this approach. If you decide to move forward, I would strongly encourage you to thoroughly test it with each upgrade the community receives.

I hope this helps!