You're not able to provide something directly in the markup. But you could provide a default value in code by editing the Components_Text.cs file (assuming you have a component called "Test") and on the ComponentInput method, before the call to InputBase you could do something like:
if (string.IsNullOrWhiteSpace(asset["field"])) asset["field"] = "Default value";
This would set the default for a field called "field". Note that it would also remove the user's ability to clear the value and save, as it would be restored next time that they edit.