Call azure function secured by AAD using client id and client secret

I have an Azure Function secured by the Azure Active Directory.

I can successfully call the application from the browser provided that I'm logged.

Now, what I want to do is to call that function from an application that doesn't use the username and password in order to authenticate but rather a trusted application client id and client secret.

I tried to use for that purpose the same application (say ApplicationA) that has been created for securing the function application. I went to the App Registrations, found the application created by configuring the Azure Active Directory authentication provider in the Function App Authentication/Authorization settings (ApplicationA) and I set a Client Secret for it.

Now I'm able to obtain a token using said application Client Id and the generated secret.

But when I'm trying to call the Function using that token I'm getting 401 error.

I tried the same operation with a newly created application in App Registration (say ApplicationB) but with the same result, I'm still getting 401.

I even tried to add a Reader Role for that application in Subscriptions but then again - 401 when calling Function with the token.

I know, there's something like API permissions in the application settings. So I tried to add the permission for ApplicationB to ApplicationA user_impersonation but it also doesn't work so that's not the way.

So how do I grant the permission for a registered Application for which I'm able to acquire a token so I can use this token to call the Function?

EDIT:

I'm using the following code for token acquisition:

using Microsoft.Identity.Client;

string ClientId = "XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX";
string ClientSecret = "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX";
string TenantId = "XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX";
string Authority = $"https://login.microsoftonline.com/{TenantId}";
string[] scopes = new string[] { "https://graph.microsoft.com/.default" };

IConfidentialClientApplication app = ConfidentialClientApplicationBuilder.Create(ClientId)
    .WithClientSecret(ClientSecret)
    .WithAuthority(new Uri(Authority))
    .Build();

AuthenticationResult result = await app.AcquireTokenForClient(scopes).ExecuteAsync();

string token = result.AccessToken;