how to insert a digital signature block for fillable form

I need to add a signature block for someone else to digital sign it and for some reason is not working. When I am adding the box on "Edit"" mode I can see the box. When I preview it, it does not show. I know I can go ahead and add a border to it but still the person can't sign. I need the gray box to appear where it has a red symbol indicating to sign here and they can sign digital. I uploaded three screenshots to explain what's going on. The first one is how it appear when I am adding the signature box, the second one is how it looks when I preview it ( blank- nothing shows), and the third one is what I am looking for. Any help? Thanks