Fail to retrieve token via pre-request script in Postman

I would like to have a script running before each request in Postman, which renews the OAuth token, if it is expired (or is about to). So I found this blog on the topic and wanted to implement that.

However, the way I usually request tokens where I work, is by sending information via the body. In this picture, you can see how it looks in Postman.

This works just fine and retrieves a token for me, when I click send. Now I tried to use this information, to modify the script from the blog, so it now looks like this:

// Refresh the OAuth token if necessary
var tokenDate = new Date(2010,1,1);
var tokenTimestamp = pm.environment.get("OAuth_Timestamp");
if(tokenTimestamp){
  tokenDate = Date.parse(tokenTimestamp);
}
var expiresInTime = pm.environment.get("ExpiresInTime");
if(!expiresInTime){
    expiresInTime = 300000; // Set default expiration time to 5 minutes
}
if((new Date() - tokenDate) >= expiresInTime) 
{
   pm.sendRequest({
      url:  pm.variables.get("base_url_token"), 
      method: 'POST',
      header: {
        'Accept': 'application/json',
        'Content-Type': 'application/x-www-form-urlencoded'
      },
      body: {
        grant_type: 'password',
        username: pm.environment.get("username"),
        password: pm.environment.get("password"),
        client_id: pm.environment.get("client_id_practice_app"),
        client_secret: pm.environment.get("client_secret_practice_app")
      }
  }, function (err, res) {
        pm.environment.set("OAuth_Token", res.json().access_token);
        pm.environment.set("OAuth_Timestamp", new Date());
        
        // Set the ExpiresInTime variable to the time given in the response if it exists
        if(res.json().expires_in){
            expiresInTime = res.json().expires_in * 1000;
        }
        pm.environment.set("ExpiresInTime", expiresInTime);
  });
}

When I run any request, it shows in the console that the pre-request script has correctly sent the auth request. But I get the following error in the response body (from the auth):

{"error":"invalid_client"}

I am almost certain that it is getting the correct Id and Secret. I have tried swapping the environment variables for hardcoded values. But I am less sure if I am setting the body correctly.

Can anyone tell me how to get this to work?