i am afraid Post-Creation tab is not a right place to do floating ip association, it is place to allow user to do some post configurations like install some packages , do update etc via shell script and it is has limitation in size.
If you want to more complex operations like associate floating ip when bootstrapping your instance, you can try use the cloud-init for cloud-init can use shell and python scripts to do much more complex tasks.
For more details about cloud-int refer to http://cloudinit.readthedocs.org/en/l...
Hope that helps!
Vic