As far as I can tell, the API does not support assigning group policies by ssid.
I should point out that the documentation for setting the policy is incomplete (or inaccurate, depending on your point of view). The valid values for devicePolicy are whitelisted, blocked, normal, group. Note that "group policy" is not a valid value. You can see these values in Meraki's python library (https://github.com/meraki/dashboard-api-python/blob/master/meraki.py#L1327).
If a device has group policies set by ssid, you can retrieve the details from the API. Here's what the response looks like:
I know how to get the info and also know how to set the correct info at the API but In my point of view if u have that option in the dashboard Meraki developers are able to implement this in short term at the API also.
I don't know if this got solved for any of you folks, but I found the proper format to send a client per SSID policy. Maybe this will help someone that finds this... took me awhile to get it. Make sure to have the correct number of SSIDs definded.
Did you get it figured out? It's just a POST in whatever language you are using. (I use perl/curl).
You POST the data as shown, "0" being the first SSID in your list in the GUI as shown above, "1" being the second etc. POST to the client provisioning api. I use version0 for this, I'm not sure it works in the v1 api.