const options = {
  method: 'POST',
  headers: {
    'x-spree-api-key': '<api-key>',
    Authorization: 'Bearer <token>',
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({product_ids: ['prod_abc123']})
};

fetch('http://{defaultHost}/api/v3/admin/collections/{collection_id}/products', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

curl --request POST \
  --url http://{defaultHost}/api/v3/admin/collections/{collection_id}/products \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --header 'x-spree-api-key: <api-key>' \
  --data '
{
  "product_ids": [
    "prod_abc123"
  ]
}
'

{
  "added_count": 1
}