MarkInvalid validation textbox using MVVM

I have a textbox that has a validation on it, in validation, it checks if its isNullOrEmpty. Validation works, but I am having difficulty if the user never selects the text box and clicks, I want the validation to run again. I was able to accomplish this in the XAML.cs file using:

Validation.MarkInvalid(cb.GetBindingExpression(dp), validationError);

      

        
        
        
      

    

Now with MVVM I am confused how can I do this from the viewmodel.
Text field in subject:

        <TextBox>
            <TextBox.Text>
                <Binding Path="LastName">
                    <Binding.ValidationRules>
                        <validationRules:IsNullOrEmptyValidationRule/>
                    </Binding.ValidationRules>
                    <Binding.UpdateSourceTrigger>PropertyChanged</Binding.UpdateSourceTrigger>
                    <Binding.Mode>TwoWay</Binding.Mode>
                </Binding>
            </TextBox.Text>
        </TextBox>

      

        
        
        
      

    

Any ideas?