This is expected behavior - Wacom STU-model (i.e., STU-540, STU-430, etc.) signature PADs are a different class of product (from normal Wacom products that are built around a create/design workflow, i.e., such as Intuos and Cintiq) and always require an application that communicates directly with them derived from our Software Development Kit (SDK) - as such they are not 'plug-and-play'.
In general we offer the following software alternatives to customers:
1. That they adapt the device’s functionality into their own applications using our SDK. Our SDK supports Java, C and VB.NET programming languages. More info -> https://developer-support.wacom.com/hc/en-us
2. A “turnkey” software option we offer for small installations for a simple sign-to-PDF-file workflow includes our sign pro PDF software. You can use the software to sign PDF documents right away at no cost, only the authoring capabilities are chargeable. We also offer our sign pro API.
3. An existing third-party integration - be sure to check for compatibility or our Wacom for Business team can assist here. If an integration does not currently exist, Wacom can work with the third-party software company to develop a solution.
More info: https://www.wacom.com/en-us/for-business/products/software
If you need more assistance related to your e-signature project please create a service ticket with Wacom Customer Care and request that it be escalated to the Wacom for Business team - we are happy to assist.