Use Javascript to force Certification signature

I am trying to force the first signature on a form to be a Certify signature using:

// Obtain the signature field object:
var f = this.getField("Signature1");
f.signatureSetSeedValue(
{
mdp: "default"
} )

I run this code On Focus for the field.

In the Debugger, I get this error:

NotAllowedError: Security settings prevent access to this property or method.
Field.signatureSetSeedValue:5:AcroForm:Signature1:Annot1:OnFocus:Action1

Any ideas on what I need to change to allow this script to run, or another way to achieve the same goal?