Clojurians Log v2

I'm trying to send a vector of integers through a multipart form, but struggling to make it work for a single value. I'm using muuntaja and malli coercion with the default configs. I create a form = new FormData() in js, then append to it like form.append('xs', 10) , form.append('xs', 20). With more than one value this works, but with just 1 it doesn't pack that into a vector. I tried these variations, form.append('xs[]', 10) / form.append('xs[0]', 10) , but in those cases it seems to just ignore it. I feel like I'm missing something obvious here, what's the intended way to do this?