With method for setting any property on any object including read only properties (for tests)

ObjectExtensions.cs

public static class ObjectExtensions
{
        public static T With<T, TProp>(this T o, Expression<Func<T, TProp>> prop, TProp value)
        {
            if (!(prop.Body is MemberExpression memberExpression))
            {
                throw new ArgumentException($"A property must be provided. ({prop})");
            }

            var propertyInfo = (PropertyInfo) memberExpression.Member;

            if (propertyInfo.CanWrite)
            {
                propertyInfo.SetValue(o, value);
            }
            else
            {
                typeof(T).GetField($"<{propertyInfo.Name}>k__BackingField",
                    BindingFlags.Instance | BindingFlags.NonPublic)?.SetValue(o, value);
            }

            return o;
        }
}

Example usage:

    public class Advisor
    {
        public Advisor(
            string advisorEmailAddress,
            string administratorEmailAddress,
            AuthorizationType authorizationType, 
            IntroducerType introducerType, 
            IEnumerable<LenderBrokerCode> lenderBrokerCodes, 
            IEnumerable<LenderBrokerCode> administratorLenderBrokerCodes)
        {
            LenderBrokerCodes = lenderBrokerCodes;
            AdvisorEmailAddress = advisorEmailAddress;
            AuthorizationType = authorizationType;
            IntroducerType = introducerType;
            AdministratorLenderBrokerCodes = administratorLenderBrokerCodes;
            AdministratorEmailAddress = administratorEmailAddress;
        }

        public string AdvisorEmailAddress { get; }
        public string AdministratorEmailAddress { get; }
        public AuthorizationType AuthorizationType { get; }
        public IntroducerType IntroducerType { get; }
        public IEnumerable<LenderBrokerCode> LenderBrokerCodes { get; }
        public IEnumerable<LenderBrokerCode> AdministratorLenderBrokerCodes { get; set; }
    }

then in your test:

var aggregator = fixture.Create<Aggregator>();
aggregator.With(a => a.AdvisorEmailAddress, "test@test.com");

This saves you from inventing dummy data for all the constructor parameters which you don't care about.