Azure AD B2C: : Error: "AADSTS50049: Unknown or invalid instance" after setting redirect URL to use b2clogin.com

I'm trying to adapt the new b2clogin.com before login.microsoftonline.com is deprecated but keep getting Error: "AADSTS50049: Unknown or invalid instance".

I'm making an authentication call from a Xamarin.Android app to an ASP.Net Core 2.1.1 web api.

I can make a successful call with PostMan using mytenant.b2clogin.com so I believe the AD B2C tenant and the web api are setup correctly.  Also, I've been using the login.microsoftonline.com successfully.

It seems as though my problem is in how to format the authority for the PublicClientApplication I'm using to acquire authorization.

//Current code to make the call:

AuthenticationResult ar;

PublicClientApplication PCA;

UIParent uiParent = new UIParent(this);

string[] Scopes = { "https://mytenant.onmicrosoft.com/api/read" };

string ClientID = "00000-000-00000-000000;

string Authority = "https://mytenant.b2clogin.com/tfp/mytenant.onmicrosoft.com/b2c_1_signupin";

PCA = new PublicClientApplication(ClientID, Authority);

ar = await App.PCA.AcquireTokenAsync(Scopes, uiParent);

Any feedback on how this should be formatted/assigned will be appreciated.  Thanks.