Follow the example code provided by shopify. You need to add the variantID (or productID if no variant), and the quantity. Refer to documentation here.
Oh by bad. I thought you are writing a fetch function. You might have miss some comma or semi-colon. If you did not change much on the product-form.js. I recommend, to add a new Dawn theme. Copy the product-form.js contents and replace the product-form.js contents in your old theme.
@made4Uo All good. I’ve just tried to do a exact copy of the product-form new Dawn theme and replaced it on my old theme and nothing had changed. So I’m not sure what else could’ve happened.