The HelloSign Text Macro adds a validation option that can be used to validate the text field while the signer is signing. The options are letters only, numbers only, phone number, bank account number, bank routing number, email, zip code, social security number, or employer identification number.
If your text field needs to be longer, you can do this by filling out the label. If it still isn't long enough, then add underscores.
The Validation field is also optional and is alphanumeric and underscore only.