A form is just a special kind of block. You can load the form (an instance of FormContainerBlock; from some sort of reference), update the SendEmailAfterSubmissionActor or CallWebhookAfterSubmissionActor properties and save the changes.

But these properties do not exist as typed properties on the FormContainerBlock. Instead, You need to set them in the form property bag. Something like this:

var writableForm = (FormContainerBlock)myForm.CreateWritableClone();
// TODO: If there are already web hook configurations on the form, add your new model to the existing one.
writableForm.Property["CallWebhookAfterSubmissionActor"] = new[]{ new WebhookActorModel() };
// TODO: Save the form.