I was enrolled inCovered CA for most of 2018, and I have a form 1095-B but not a 1095-A. I also have additional health coverage to add from August 2018 onward. How can I

Your question got cut off. Can you reply with more detail?

Just indicate the months that you were covered in the Healthcare section. If you were not on a Marketplace plan, say no. Then you are done.