First, you will need to contact firstname.lastname@example.org to turn on the HTMLOverride feature flag. By default, the Flatfile importer does not parse the options you have chosen in the customization options as code. Enabling the HTMLOverride feature flag allows you to do some limited HTML coding.
We advise you to be careful when using this feature. Too much customization may cause the importer to work incorrectly. However, small changes, like adding paragraph breaks, should be okay.