You need to script the insertion of the signature using the Insert from Device [<table::field>; Type: Signature] script step in FileMaker Pro v13.
Once you've created the script you can assign it to a button (or make the container field the button). You also need to remove access to your container field in Browse mode so users can't click into the field which brings up the standard iOS selctor which is allows users to insert non signature items (photos etc).
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Fantastic.... Thank you very much