Sending Object Arrays as JSON using FormData

I am sending a FormData object to an endpoint. A phone number needs to be formatted as this JSON:

"phone": [{"type":"main", "value":"#"}, ...] or it gets rejected. A single object with a two-pair of keys and values in an array.

const doStuff = () => {
  const formData = new FormData()

  **Have tried below for setting key/value of phone object**

  // Attempt 1
  formData.set('phone', [{ type: 'main', value: '313-555-2121' }])
  // Returns:
  "phone":"[Object Object]"

  // Attempt 2
  formData.set(
    'phone',
    JSON.stringify([{ type: 'main', value: '313-555-2121' }])
  )
  // Returns
  "phone":"[{\"type\":\"main\",\"value\":\"313-555-2121\"}]"

  // Format as single "fields" object and stringify (results in fields: {...stuff}), API needs this.
  const formattedForApi = JSON.stringify({fields: Object.fromEntries(formData.entries())})

  // MAKE POST REQUEST...
}

The API errors on both of my attempts above. Complaining of an invalid first value which needs to be "main". Am I missing something with how stringify is affecting the data that is actually being sent?

For those wondering, the API is Podio

Digging into the PHP SDK code, it seems you're supposed to send the fields as plain old JSON and definitely not double-encoded

const formattedForApi = JSON.stringify({
  fields: {
    phone: [
      {
        type: "main",
        value: "313-555-2121",
      },
    ],
  },
});

fetch(`/item/app/${app_id}/`, {
  method: "POST",
  body: formattedForApi,
  headers: {
    authorization: `OAuth2 ${token}`,
    "content-type": "application/json",
  },
});