msgraph-dotnet-v5-rc-custom-auth-flow

CustomAuthFlow.cs

public class TokenProvider : IAccessTokenProvider
{
    public Task<string> GetAuthorizationTokenAsync(Uri uri,
            Dictionary<string,
            object> additionalAuthenticationContext = default,
            CancellationToken cancellationToken = default)
    {
        var token = "token";
        //get the token and return it in your own way
        return Task.FromResult(token);
    }
    public AllowedHostsValidator AllowedHostsValidator {get;}
} 

var accessTokenProvider = new BaseBearerTokenAuthenticationProvider(new TokenProvider());
var graphServiceClient = new GraphServiceClient(accessTokenProvider);