How to set the "create new pool" option in stock http iapp to auto populate for fqdn as the default

The way that I normally do this is to do the presentation part first and then do the implementation. For presentation, you probably want to change section pool>table members

table members {
            editchoice addr display "large" tcl {
                package require iapp 1.1.1
                return [iapp::get_items ltm node]
            }

And to change the implementation side, find where it mentions pool__members and modify the commands:

 array keys: $new_pool,$no_pool
array set pool_arr {
    1,0 { [iapp::conf create ltm pool ${app}_pool \
          [iapp::substa pool_ramp_pga_arr($advanced,$do_slow_ramp,$do_pga)] \
          [iapp::substa pool_lb_queue_arr($advanced,$is_edge,$tcp_queuing)] \
          [iapp::substa monitor_arr($new_pool,$new_monitor,$advanced)] \
          [iapp::pool_members $::pool__members]] }
    0,0 { [expr { $::net__server_mode ne "tunnel" ? \
          $::pool__pool_to_use : $::pool__pool_to_use_wom }] }
    *   { none }
}

I find it easiest to work out the tmsh command that you want to use first and then implement that. It's easier than it looks, honest!