Create a node based on an event

if the goal is to add the node to a pool, then yes, you'll need something like iCall to do that. If you just want to forward the request to the node and not have BIG-IP "manage" that node as a pool member with health monitors, then you can just do that straight up in an iRule. Assuming you are handling the fqdn extraction from the uri, a header, or the payload, it's pretty basic:

when RULE_INIT {
  set static::ldns "x.x.x.x"
}

when HTTP_REQUEST {
  set node_fqdn "your code to get fqdn here"
  set node_ip [RESOLV::lookup @$static::ldns -a $node_fqdn]
  if {$node_ip eq ""}{
     didn't resolv, send to default pool?
  } else {
       multiple ip's possibly returned, this covers string and list scenario
      node [lindex $node_ip 0]
  }
}