UiPath Custom Activity

Try this( modified)

public class SimpleFormula : CodeActivity
    {
        [Category(“Input”)]
        [RequiredArgument]
        public InArgument<SecureString> Input { get; set; }

        [Category("Output")]
        public OutArgument<String> output { get; set; }

        protected override void Execute(CodeActivityContext context)
        {
            var strSecPassword = Input.Get(context);
            var strPassword = new System.Net.NetworkCredential(string.Empty, strSecPassword).Password;
            output.Set(context, strPassword);
        }
    }