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My suggestion would be to add Signature widget to your form.
Once you print out the PDF you should have the signature field line shown, but it will allow you to also capture the signature on your form directly.
You can see it here: E-Signature
Alternatively to that you could use Add Text option in the PDF designer and then resize it to give you as much space as you need.
A quick demo can be seen here:
To add the underline, I just pressed and hold the underline key on my keyboard, so you can make it as big as you want it, and as you can see it can be centered or moved to any position you want.