Currently it is not possible to authenticate at Symantec Drive Encryption's Pre-Boot authentication screen, called Bootguard, using the Microsoft Surface Pro 4 virtual keyboard.
Symantec Corporation is committed to product quality and satisfied customers. This Feature Request is currently being considered by Symantec Corporation to be addressed in a forthcoming version of the product.
Technical Support filed a Feature Request to add this product feature. Note that an feature request is exactly that, a request. There is no committed date for this request from the Endpoint Encryption Product Management team, nor from the Endpoint Encryption Engineering team at this time.
Please be sure to refer back to this document periodically as any changes to the status of the request will be reflected here.