First you need to make a new app in your orderform configuration. To do that:
Send a GET request here: https://{{vendor}}.vtexcommercestable.com.br/api/checkout/pvt/configuration/orderForm
Copy the response and change the apps section. Add a new object there
"apps": [
{
"fields": [
"YOUR_FIELD"
],
"id": "YOUR_APP"
}
],
Send a POST request on the same endpoint with your updated payload.
After that, you can add custom data using the next method from checkout js:
vtexjs.checkout.setCustomData({app: 'YOUR_APP', field: 'YOUR_FIELD', value: 'VALUE_HERE'});