Go to Settings > Edit, then in the "Custom Code" section, you can link your font host. Then, define which font to use for headings and paragraphs.
NOTE: The font names are case sensitive, so if the font family is sans-serif, the custom font name should be Sans-Serif, not Sans Serif.
These fonts will then be available to choose from in the font dropdowns in the builder and will be loaded on ConvertFlow's hosted landing pages.