Issue with Google Service Account API

Questions
1

Describe the problem/error/question

Im having an issue, as all the time I receive 403 from application which is set up in Google Cloud Vertex. According to the received and found information I set up:

  1. Method: GET
  2. URL: url to my app
  3. Authentication: Predefined Credential Type
  4. Credential Type: Google Service Account API
  5. My configured account: after providing region, service account email and private key I have information that connection tested successfully. And now Im not sure whether the problem is on my external app ( where all permissions are granted) or mayby here params not added to request?

What is the error message (if any)?

Full message

403 - "\n<html><head>\n<meta http-equiv=\"content-type\" content=\"text/html;charset=utf-8\">\n<title>403 Forbidden</title>\n</head>\n<body text=#000000 bgcolor=#ffffff>\n<h1>Error: Forbidden</h1>\n<h2>Your client does not have permission to get URL <code>/</code> from this server.</h2>\n<h2></h2>\n</body></html>\n"