Yes, you can do that with the following C# snippet:
Text textTab1 = new Text { ConcealValueOnDocument = "true", Required = "true", Locked = "true" /* add additional properties... */ };
However, I find this odd. If the recipients cannot modify, you can set the value to be "**********" and use a custom field for any sensitive information you want to send (so it's metadata in the envelope but not visible to anyone).
ConcealValueOnDocument was intended for situations where you have signer 1 enter their SSN and signer 2 enter their SSN, but you don't want them to see eachother's SSNs... (for example).
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