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Is this what you want? This is a png, transparent BG.
View attachment 35367
And yes, you use layers, but if you have a new autograph, you'll have to load that section into Photoshop and I would suggest a closeup of course. You might be able to make an action for this.
When I didn't hear back from you, I deleted the psd. I could guess what I did, but I'd rather be certain by doing it, which I can't do immediately. I will say this. It is no one step solution. It involves masking, inverting, levels or curves adjustment, and I didn't get to refine edge smoothing on the example I posted.
So it's important to know your level of experience with Photoshop before I try to outline the steps.