How to Create/Modify iApp via API?

iControl-REST is not fully idempotent, so you may run into a problem posting back what you get. You may also have a character interpretation issue, where Tcl is consuming the curly braces in your block of json. Look in /var/tmp/scriptd.out for a log of the tmsh statement that failed, and perhaps add more debugging statements to convince yourself that the values in MyCoolSite.json are showing up in the Tcl context.