Web Forms :: Adding New Error Message To Validation Summary Control?

In the code behind based on a condition i want to add a error message to the Validation Control.

I tried the below.

'Call function to add error message.
protected void AddErrorToValidationSummary(string errorMessage)
{
CustomValidator custVal = new CustomValidator();
custVal.IsValid = false;
custVal.ErrorMessage = errorMessage;
custVal.EnableClientScript = false;
custVal.Display = ValidatorDisplay.None;
this.Page.Form.Controls.Add(custVal);
}