I don't see your image, but all of the Title templates do this by default. If you want to create your own from scratch, launch Motion 5. From the Template Browser, select the "Final Cut Title" template. This has an automatic transparent background.
If you don't want to create a template and just use one of the ones that come with FCPX, go to the Title Template Browser in the tool bar, and look for the Custom or Basic Title templates. All the controls you need are in the Inspector.
Here's a two part pair of articles that walks through basic titles in FCP X.