Adding a source IP address restriction to an existing LTM policy rule using tmsh

I have a rule in a policy that restricts access to a URL based on the incoming source IP. I've tried both add and modify using a tmsh command to add an additional IP address but both have resulted in wiping out the list of IPs and only leaving the new address.

What is the correct method to append, or add, an IP address to the list via tmsh? We're on version 13.1.1.4