Here's an example from iax.conf on one of my Acid servers with three nodes:

register=28445:999999@register.allstarlink.org
register=29043:999999@register.allstarlink.org
register=27303:999999@register.allstarlink.org

This is from a box installed about a year ago. And of course here I've replaced the node password with 999999.

As to XIPAR, it's in iax_registrations_custom.conf

register=27783:999999@register.allstarlink.org

(This is from a XIPAR install about a month old)