Use Master User for auto login to Power Bi Embedded (for customers) in Xamarin Forms app

Good day to all of you. I have a problem with probably not fully understanding how to embed PBI report in my Xamarin app. 
I got my token and I used Master User for this, but when I put my EmbedUrl in Source proprety of WebView, i get a message: "Please sing in to view this report". This is a problem because my customers don't have PowerBi account and I want them to be able to see this report. Problem might be in my headers I use to get token, so below is code I use

string url = "https://login.microsoftonline.com/" + tennant_id + "/oauth2/token"; //url to which I POST to

IEnumerable<KeyValuePair<string, string>> parameters = new List<KeyValuePair<string, string>>()
{
new KeyValuePair<string, string>("authenticationType", "MasterUser"),
new KeyValuePair<string, string>("name", "<name>"),
new KeyValuePair<string, string>("workspaceId", "<workspaceId>"),
new KeyValuePair<string, string>("grant_type", "password"),
new KeyValuePair<string, string>("scope", "https://analysis.windows.net/powerbi/api/.default"),
new KeyValuePair<string, string>("resource", "https://analysis.windows.net/powerbi/api"),
new KeyValuePair<string, string>("authorityUrl", "https://login.microsoftonline.com/organizations/"),
new KeyValuePair<string, string>("urlPowerBiServiceApiRoot", "https://api.powerbi.com/"),
new KeyValuePair<string, string>("client_id", "<clientID>"),
new KeyValuePair<string, string>("username", "<username>"),
new KeyValuePair<string, string>("password", "<password>"),
new KeyValuePair<string, string>("client_secret", "<clientSecret>")
};

 After I get access token, I use it in GET request to 

https://api.powerbi.com/v1.0/myorg/reports/<report_it>

 and I pass AuthenticationHeader as 

client.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Bearer", accessToken);

Then I get my embed url which I use in WebView Source property but I am not logged in. When I do login i see this report, but otherwise I can't. Where am I making a mistake/mistakes?