Some example URL’s which have strokes below

https://res.cloudinary.com/dmicpnvb0/image/upload/bo_20px_solid_White,l_text:Sacramento_50_stroke:The%20yoga%20pose%20you%20avoid%20the%20most%20you%20need%20the%20most./05d21216-6c20-49a2-b8c1-96bd29fc67c1.jpg

The transformation object i sent was

    "transformation": [
        {
            "overlay": {
                "font_family": "Sacramento",
                "font_size": 50,
                "text": "Some content for the image",
                "stroke": "stroke"
            },
            "border": {
                "color": "White",
                "width": "20"
            }
        }
    ]

The key being in the overlay section you need the “stroke”: “stroke”

and then a seperate section called border, which is where you define the overlay.