PowerBiController.cs

/// <summary>
/// Retrieves the access token required to access the Power BI Pro account data.
/// </summary>
/// <returns>Access token required to access Power BI reports</returns>
/// <response code="200">OK</response>
/// <response code="401">Unauthorized</response>
// GET: api/reports/accessToken
[HttpGet]
[Route("api/reports/accessToken")]
public async Task<IHttpActionResult> GetAccessToken()
{
    var accessToken = await GetAccessTokenInternalAsync();

    if (accessToken == "")
    {
        return Unauthorized();
    }
    return Ok(accessToken);
}
        
/// <summary>
/// Retrieves the access token required to access the Power BI Pro account data.
/// </summary>
/// <returns>Access token required to access Power BI reports or empty string on error.</returns>
private async Task<string> GetAccessTokenInternalAsync()
{
    // Check for existing and valid access token
    if (powerBiAccessToken != "" && ValidateToken(powerBiAccessToken))
    {
        return powerBiAccessToken;
    }

    // We might need to change the following code for .NET Core in near future. 
    // Refer https://community.powerbi.com/t5/Developer/Embed-Power-BI-dashboard-in-ASP-Net-core/m-p/284314#M8436
    try
    {
        // Create a user password credentials.
        var credential = new UserPasswordCredential(powerBiConfiguration.UserName, powerBiConfiguration.Password);

        // Authenticate using created credentials
        var authenticationContext = new AuthenticationContext(powerBiConfiguration.AuthorityUrl);
        var authenticationResult = await authenticationContext.AcquireTokenAsync(powerBiConfiguration.ResourceUrl, powerBiConfiguration.ClientId, credential);

        if (authenticationResult == null)
        {
            return "";
        }
        powerBiAccessToken = authenticationResult.AccessToken;
        return authenticationResult.AccessToken;
    }
    catch (Exception e)
    {
        Console.WriteLine("Failed to fetch Power BI access token, exception details: ", e);
    }
    return "";
}

/// <summary>
/// Validates the given access token.
/// </summary>
/// <param name="token">Access token to be checked for validity (required)</param>
/// <returns>True if token is valid, false otherwise.</returns>
private bool ValidateToken(string token)
{
    // Code reference: https://github.com/dream-365/OfficeDev-Samples/blob/master/samples/Office365DevQuickStart/OAuth2-basic/JsonWebTokenValidator.cs
    bool isValid = true;
    try
    {
        string stsDiscoveryEndpoint = "https://login.microsoftonline.com/common/v2.0/.well-known/openid-configuration";
        SecurityToken jwt;
        ConfigurationManager<OpenIdConnectConfiguration> configManager =
            new ConfigurationManager<OpenIdConnectConfiguration>(stsDiscoveryEndpoint,
            new OpenIdConnectConfigurationRetriever());

        OpenIdConnectConfiguration config = configManager.GetConfigurationAsync().Result;

        TokenValidationParameters validationParameters = new TokenValidationParameters
        {
            ValidateAudience = false,
            ValidateIssuer = false,
            IssuerSigningKeys = config.SigningKeys,
            ValidateLifetime = true
        };

        JwtSecurityTokenHandler tokendHandler = new JwtSecurityTokenHandler();
        tokendHandler.ValidateToken(token, validationParameters, out jwt);
    }
    catch (Exception)   // Token invalid/expired
    {
        isValid = false;
    }

    return isValid;
}

Web.config

pbiConfig.AuthorityUrl = "https://login.windows.net/common/oauth2/authorize/";
pbiConfig.ResourceUrl = "https://analysis.windows.net/powerbi/api";
pbiConfig.ApiUrl = "https://api.powerbi.com/";
pbiConfig.ClientId = "Provide application/client ID of the app registered in Azure AD here";
pbiConfig.GroupId = "Provide Power BI workspace/group ID here";
pbiConfig.UserName = "Provide Power BI service (app.powerbi.com) user name here";
pbiConfig.Password = "Provide Power BI service (app.powerbi.com) password here";

@rrohitesh sorry I don't have access to it anymore.