If I am not wrong, you are missing something here. I often try like this and it works.
fetch('https://example.com/api/collections/save/consumers?token=12345678901112131415', {
method: 'post',
headers: { 'Content-Type': 'application/json' },
body: JSON.stringify({data: {
"ass_consumer": "_id of your newly created account",
// the rest fields
}})
})
.then(e => e.json())
.then(res => console.log(res));