When you are asked did you have HDHP coverage at any time in 2018, click on "yes".

This will expose three new lines. Pick the last one - I had different coverage at different times.

The next screen will ask you for the coverage for each month. Choose "None" for January to June. Whether or not you add July to this list depends on what day of the month you started your HDHP coverage. If you started the HDHP AFTER July 1st, then choose "None" for July (yes, it's the coverage on the first day of the month that matters). Otherwise, see the next paragraph for what to check.

Then for every month you have coverage, choose either Self or Family, depending on what your HDHP policy is. Self is you only, and Family is you and at least one other person (any person: spouse, child, the mailman, anyone).