Use the ip of whichever CUCM server you prefer it to register to. Alternately, use a domain name in that field and create SRV records in that domain which point to your call manager servers with your chosen priority/weight configuration. This allows the device to fail to a different call manager if the first one goes down.
You make an SRV record in dns in the zone you entered as proxy in your SPA (voip.domain.com, for example) and then the SRV record would be like _sip._udp.voip.domain.com with your SIP port (5060 usually), and a priority and weight for each call manager server based on the order you want the device to try to register to them. Lower number is higher priority, think of weight as load balancing within servers with the same priority number. Easiest if you have three CUCM and you want the device to register to them in one specific order, top down, is to set them as priority 1,2, and 3 and weight 100 for all.
On the spa you would have to set use dns srv to yes and the auto prefix based on if you're using the default (_sip._udp.) and whether you manually included it in your proxy address or just left it as voip.domain.com or domain.com or whichever domain you created your SRV records in.
