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.