I am trying to send data to google apps script so that it will add that data onto a google slides presentation that will be created in the users google drive. I am using the OAuth2 library.

https://github.com/googleworkspace/apps-script-oauth2

I am sending a fetch request to my web app url (execute as user, accessible to anyone with google account). However I keep on getting a CORS error:

Access to fetch at ‘https://script.google.com/macros/s/AKfycbzCdWHQn3AOxBsICRFxVqJ8DBvNoqUGX47Z1_svLZ-SRWE706WzmcSq3hWFwVzuWCjm/exec’ from origin ‘http://localhost:10003’ has been blocked by CORS policy: No ‘Access-Control-Allow-Origin’ header is present on the requested resource. If an opaque response serves your needs, set the request’s mode to ‘no-cors’ to fetch the resource with CORS disabled.

**Here is my code
**

    async function sendData() {
        const jsonData = [{
                "subject": "The girl",
                "predicate": "ran home"
            },
            {
                "subject": "The cat",
                "predicate": "wore a hat"
            },
            {
                "subject": "The tree",
                "predicate": "fell down"
            }
        ];

        try {
            const response = await fetch(
                'https://script.google.com/macros/s/AKfycbxFkvMAEey-UPOWSJSqMMQBqLS0hPcUkYQRLTKAOPLuC4k-DU28sS8pVeRFhyMwzGfs/exec', {
                    method: "POST",
                    headers: {
                        "Content-Type": "application/json",
                    },
                    body: JSON.stringify(jsonData),
                    credentials: "include" // Ensure credentials are included for cross-origin requests
                });

            const responseText = await response.text();
            console.log("Data response:", responseText);

            // Check if the response contains an HTML redirection link
            if (responseText.includes('Authorize')) {
                // Redirect to the authentication URL
                const parser = new DOMParser();
                const doc = parser.parseFromString(responseText, "text/html");
                const redirectUrl = doc.querySelector("a").href;
                window.location.href = redirectUrl;
                return;
            }

            const data = JSON.parse(responseText);
            console.log("Success:", data);
            alert("Data sent successfully: " + JSON.stringify(data));
        } catch (error) {
            console.error("Error:", error);
            alert("Error sending data: " + error.message);
        }
    }

**Here is my apps script code:
**

function doPost(e) {
  Logger.log("doPost function ran");
  
  var service = getOAuthService();
  if (service.hasAccess()) {
    Logger.log("User has access");
    var accessToken = service.getAccessToken();
    
    try {
      Logger.log("acess Token: " + accessToken);
      
      var response = UrlFetchApp.fetch('https://www.googleapis.com/drive/v2/files?maxResults=10', {
        headers: {
          Authorization: 'Bearer ' + accessToken
        }
      });
      Logger.log("Drive API Response: " + response.getContentText());
      
      var jsonData = JSON.parse(e.postData.contents);
      
      var presentation = copyAndModifyPresentation(jsonData);
      var url = presentation.getUrl();
      Logger.log("Presentation URL: " + url);
      
      return ContentService.createTextOutput(JSON.stringify({ message: 'Presentation created with URL: ' + url }))
          .setMimeType(ContentService.MimeType.JSON);
    } catch (err) {
      Logger.log("Error: " + err.message);
      return ContentService.createTextOutput(JSON.stringify({ error: 'Invalid JSON: ' + err.message }))
          .setMimeType(ContentService.MimeType.JSON);
    }
  } else {
    Logger.log("User does not have access. Need to authorize.");
    var authorizationUrl = service.getAuthorizationUrl();
    return ContentService.createTextOutput(JSON.stringify({ status: "unauthenticated", authUrl: authorizationUrl }))
        .setMimeType(ContentService.MimeType.JSON);
  }
}

Thank you

I have tried using no-cors but the data is only posted if the user is already signed into a google account. To prompt the user to sign in, I can’t have the mode set as no-cors, because I need a response which would contain the authorization URL.

I also tried setting up a PHP proxy server but it gives me a 401 error code
POST http://localhost:10003/wp-content/themes/twr/handle-google-auth.php 401 (Unauthorized)