A. If you wanted to just add a line under where the signature will be you can do something like this:

signature.Text3.Type = Pronto.Signature.TextTypes.TextPropertyDefined 'THIS LINE IS OPTIONAL SINCE TextPropertyDefined IS THE DEFAULT
signature.Text3.Placement.Field = "pronto_signature1"  'This being the same field that your signature is on (or you can just use raw coordinates)
signature.Text3.Text = "______________________________________________"
signature.Text3.Placement.Y = 0 'You might need to adjust the (X,Y) to slide under your signature better
signature.Text3.Placement.X = 0

(Note: You could use Text1, Text2 or Text3 for this)