Batboard

I have a question about a high power VHF Spectra, which I would like to make secure capable. This unit is not currently secure capable, I suspect in requires an addition board where the KVL is used to load it at the base. I have tried to load it from the control head, but key fails. There is a rubber plug next to the power cable at the base, but when attempting to remove it, it goes all the way through. I thought loading directly at the base would have been more appropriate, but its missing the adapter. The radio accepts to the secure options via the RSS, but when attempting to load key via control head it fails.

Does anyone have any thoughts on this, and if the radio is a non-secure radio, can an additional board be added to accept the KVL load from the base?

Thank you all for your posts in advance.