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You can transfer the error messages from FluentValidation to the ModelState for a complex property using the following steps:

1. Create a custom validation attribute that inherits from ValidationAttribute.
2. Override the IsValid method of the ValidationAttribute and implement the logic to validate the complex property using FluentValidation.
3. If the validation fails, use the context argument of the IsValid method to fetch the error messages from the FluentValidation ValidationResult.
4. Add the error messages to the ModelState by calling the AddModelError method of the ModelStateDictionary.

Here is an example implementation of the custom validation attribute:

public class ComplexPropertyValidationAttribute : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        if (value == null)
        {
            return true;
        }

        // Create the FluentValidation validator for the complex type
        var validator = new ComplexTypeValidator();

        // Validate the complex type and fetch the result
        var validationResult = validator.Validate(value);

        if (!validationResult.IsValid)
        {
            // Fetch the error messages from the ValidationResult and add them to the ModelState
            foreach (var error in validationResult.Errors)
            {
                var errorMessage = $"{error.PropertyName} - {error.ErrorMessage}";
                // Add the error message to the ModelState using the complex type property name
                ModelState.AddModelError(string.Empty, errorMessage);
            }

            return false;
        }

        return true;
    }
}

In this example, you can use the ComplexTypeValidator class to validate the complex property with FluentValidation. You will also need to make sure that the complex type is decorated with the ComplexPropertyValidationAttribute to trigger the validation when it is passed to a controller action.