DLLs must be pre-installed on the client machine. The terminal service does not do any provisioning of DLLs.
To load third-party plugin DLLs into WorkPlace when the RDP GTS session starts, select the Enable third-party plugin DLLs checkbox.
Enter the DLLs to load, separating them with commas. By clicking on the {variable} button, you can select pre-defined variables from the pop-up list: