fontFamily

You can specify the font file to be used during the image creation with the fontFamily property.

Either specify the URL to a publicly hosted font file, or a short identifier referencing a font in the Library.

{
    "template": "sample-text",
    "sizes": [
        {
            "width": 1080,
            "height": 1080,
        }
    ],
    "elements": {
        "headline": {
            "text": "This is the headline",
            "fontFamily": "https://www.freefonts.com/bigbold.ttrf"
        },
        "subheading": {
            "fontFamily": "jxnfTsjeWI"
        }        
    }
}