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.
Can this do what you're looking for?
Regards,
RIchard.