HttpClient OAuth "No user found matching username"

I'm working on a console application to automatically interact with an API that needs and OAuth authorized user with a specific set of claims. If I use a browser to log in with a username and password everything works fine, but if I try to connect with a HttpClient I get the error:

"No user found matching username:"

var handler = new HttpClientHandler() { AllowAutoRedirect = true };
var client = new HttpClient(handler);
TokenResponse tokenResponse = client.RequestPasswordTokenAsync(new PasswordTokenRequest
{
    Address = "https://localhost:44328/connect/token",
    ClientId = "clientid",
    ClientSecret = "secret",
    Scope = "api1 roles",
    UserName = "username",
    Password = "password"
}).Result;

I'm using Duende.IdentityModel for the RequestPasswordTokenAsync. The message appears in the server log., followed by this line: User authentication failed:

invalid_username_or_password, details: { "ClientId":"clientid", "ClientName":"clientname", "GrantType":"password", "Scopes":"api1 roles", "AuthorizationCode":null, "RefreshToken":null, "UserName":"username", "AuthenticationContextReferenceClasses":null, "Tenant":null, "IdP":null, "Raw":{"grant_type":"password","username":"username","password":"REDACTED","scope":"api1 roles"},"$type":"TokenRequestValidationLog"}

Why do the credentials work in browser but not when called by the client?