How to send credentials to ASMX service with Fiddler?

Hi

I followed this tutorial https://www.guru99.com/security-web-services.html to add basic authentication to an asmx webservice running under https (read only data).

In short i added

public class AuthHeader : SoapHeader
{
public string UserName;
public string Password;

}

My method is surrounded with

[SoapHeader("Credentials")]

and the method includes

if (Credentials.UserName == "username") && (Credentials.Password == "password"){
// Get Data
}

When i run the service in a browser i see

POST /MyServices.asmx HTTP/1.1
Host: localhost
Content-Type: text/xml; charset=utf-8
Content-Length: length
SOAPAction: "http://tempuri.org/GetData"

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
<soap:Header>
<AuthHeader xmlns="http://tempuri.org/">
<UserName>string</UserName>
<Password>string</Password>
</AuthHeader>
</soap:Header>
<soap:Body>
<GetData xmlns="http://tempuri.org/" />
</soap:Body>
</soap:Envelope>

I would like to use Fiddler to retrieve the data, so i create the POST request and the soap:Envelope to the header but it returns no data (probably due to the username/password not sent correctly).

I then read up on clicking Tools > Text Wizard to enter the username and password in the format myusername:password which showed the value in BASE64 added

Authentication: Basic BASE64StringForUsernamePassword

but again this didnt work (again probably due to me doing something wrong).

Does anyone know how i could send a request using Fiddler with a username and password? Or have i missed something somewhere else?