Microsoft Azure - sending emails with attachments

I am trying to send email with attachment (zip file). I managed to do so, but now my main problem is that I need code to get access_token, if I try GET request to get the code

https://login.microsoftonline.com/{tenantId}/oauth2/v2.0/authorize?client_id={clientId}&response_type=code&redirect_uri=https%3A%2F%2Flocalhost&response_mode=query&scope=offline_access%20Mail.ReadWrite%20Mail.send&state=111111

I get error saying that I need to sign up, that's a problem because I can't do so from my app.

I tried this method I found

{
   client_id: clientId,
   scope: 'https://graph.microsoft.com/.default',
   grant_type: 'client_credentials',
   client_secret: clientSecret,
}

and I get access token but I cannot send a mail with video with that access token as I get http 401, my app has these permissions Mail.ReadWrite, Mail.Send, User.Read

Note that, you need to grant permissions of Application type while generating access token using client credentials flow.

I registered one Entra ID application and granted Mail.Send permission of Application type as below:

Now, I generated access token using client credentials flow with below parameters:

POST https://login.microsoftonline.com/tenantId/oauth2/v2.0/token
grant_type:client_credentials
client_id:appId
client_secret:secret
scope: https://graph.microsoft.com/.default

Response:

You can decode the above token in jwt.ms website and check roles claim for permissions:

Using this access token, you can send the mail with attachments successfully like this:

POST https://graph.microsoft.com/v1.0/users/userId/sendMail
Content-type: application/json

{
  "message": {
    "subject": "Meet for lunch?",
    "body": {
      "contentType": "Text",
      "content": "The new cafeteria is open."
    },
    "toRecipients": [
      {
        "emailAddress": {
          "address": "user@mail.com"
        }
      }
    ],
    "attachments": [
      {
        "@odata.type": "#microsoft.graph.fileAttachment",
        "name": "attachment.txt",
        "contentType": "text/plain",
        "contentBytes": "SGVsbG8gV29ybGQh"
      }
    ]
  }
}

Response:

Reference: user: sendMail - Microsoft Graph v1.0 | Microsoft