I'm not sure that it is possible with the current implementation.
The site is using the DGT Live Chess software which is really meant for broadcasting games. I could be wrong, but I can't find a way to set the clocks programatically using this (as opposed to the Rabbit Plugin).
The DGT board provides the feature to display the remaining time on a connected DGT clock. For example, the Acid Ape Chess App or Lucas Chess make use of this feature.
When playing on chess.com, a connected DGT clock does not show the remaining time. It would be of great value if this feature would also be supported for playing on chess.com.