Creating SSIDs by API POST request that seems not available yet unfortunately.
Alternative idea for you to deploy SSIDs for your wireless networks would be using the configuration template.
You can create a template and bind it to all of target networks which would reduce your operation for setup and any changes in future much if all of wireless networks are designed and configured same.
KB: https://documentation.meraki.com/zGeneral_Administration/Templates_and_Config_Sync/Managing_Multiple...
Hope this would help some!
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