Can't Add Body to Post request for session key

Exception: Request failed for https://_mebabsehomeurl_returned code 400. Truncated server response: {"errors":{"username":"value must be a non-blank string."}} (use muteHttpExceptions option to examine full response).
This is a response I get on Google Apps Script, writing a function to populate a spreedsheet.
I've been through the forums and other questions but I seem to not be able to make the request succcessfully.
Metabase version =

I'm able to get a key on Postman and browser. But generating one for a function seems impossible. I can run other get and post requests on dashboards and cards easily. So I understand my way around.

function SessionsKey() {
  var obj = { 'username':'_email_', 'password': '_password_'} 
  var options = {
      'method': 'post',
      'headers': {'Content-Type': 'application/json'},
      'data': obj,
  var response = UrlFetchApp.fetch('https://_metabasehomeurl_/api/session', options);
  var responseData = JSON.parse(response);

I've swapped data, for json body with no avail. Removed the double quotes and still won't work

The error seems to be clear and that it seems you are supplying an empty username ... can you perform some prints along the way to see what date you are passing

The username is at the 'email' position. It displays this error when it can't get the body object of the request, as I've seen on postman.
The body object doesn't seem to be readable for some reason.

I’m pretty sure that the “data” key in the json is not the correct way of sending data in a api call body, that’s why it’s failing

Thanks for your reply.
My initial hunch was this so i tried, body / data and json as the key. Also alternatively tied it to json.stringify() on the three options after removing quotation maks, to no avail.

I found a solution to this. Thank you everyone for you assistance.

The way to request is using payload keyword.
Unfortunately, this isn't clear on goole documentation
I'll post my solution for anyone who needs assistance.
FYI, I was making a request to Metabase for a session key.

function SessionKey() {
  var obj = { username:"_EMAIL_", password: "_PASSWORD_"} 
  var options = {
    method: "POST",
    payload: JSON.stringify(obj),
    headers: {"Content-Type": "application/json"}

  var response = UrlFetchApp.fetch("_URL_", options );
  var responseData = JSON.parse(response);