As of the moment, there isn't an integrated way to plug in a signature pad to capture a signature for customers' sales. While the option is not yet available, I suggest letting our product developers know about your request by sending feedback. I'll show you how.
Go to Help.
Select Send Feedback Online.
Click Product Suggestion.
Under Product Area, choose Customers & Sales.
Enter a few words describing how you wanted to have the option to capture a signature while creating the invoice.