Not knowing your currently used software Version I simply post a link to the latest 11.12.6 VBP Configuration guide from our Polycom support page => here <=
The pre-requisite for any certificate are (source page 96):
A file must be in Base64, PEM-encoded format and have the file extension, ".pem".
You will need both a certificate and a private key and may need to contact your certificate issuer for further support.
The community's VoIP => FAQ <= contains this post here:
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
Polycom Global Services
Notice: This community forum is not an official Poly support resource, thus responses from Poly employees, partners, and customers alike are best-effort in attempts to share learned knowledge. If you need immediate and/or official assistance please open a service ticket through your proper support channels. Please also ensure you always check the VoIP , Video Endpoint , Skype for Business , PSTN or RPM FAQ's