How to call a REST API from code, I only get AuthenticationException, UntrustedRoot

I am quite new to REST services, and I am trying to call a REST service from a colleague, secured only by Username and Password.

Using Postman everything is fine, I have a GET to the URL, I select "Basic Auth", set Username and Password and get the expected result.

Now I tried it via code:Calls to other websevices per code without user/pw work just fine, but here I just get an Exception:

public async Task<List<string>> MinimalProblemShowcase(){    string authenticationString = $"{clientId}:{clientSecret}"; // no typo, values are copy/pasted    string base64EncodedAuthenticationString = Convert.ToBase64String(Encoding.UTF8.GetBytes(authenticationString));    _client.DefaultRequestHeaders.Add("Authorization", "Basic " + base64EncodedAuthenticationString);    try    {        var httpResponse = _client.GetAsync(Url).Result;        // ToDo: evaluate httpResponse - but we don't get here...    }    catch (Exception ex)    {        // ...instead we drop here:        // System.Net.Http.HttpRequestException:         // The SSL connection could not be established, see inner exception.        // --->System.Security.Authentication.AuthenticationException:         // The remote certificate is invalid because of errors in the certificate chain: UntrustedRoot    }    return [];}

As indicated, I get

System.Net.Http.HttpRequestException: The SSL connection could not be established, see inner exception. ---> System.Security.Authentication.AuthenticationException:     The remote certificate is invalid because of errors in the certificate chain: UntrustedRoot

I am not aware of using certificates explicitely, but if so, it's on the same machine...

I suspect, it is just a minor detail I am missing, but what...?Thank you for any hint or help!