Workaround:
Use a deep link to pull the full Org-wide JSON data, which includes the specific Network IDs.
https://create.meraki.io/build/deep-links/
Here is the important bit:
~~~~~
Network ID within a Combined Network
Get network IDs for use with some of the Meraki Dashboard API endpoints, such as Return the Policy Assigned to a Client (Click-through splash page). This endpoint requires the specific wireless network ID within a combined network. Since the normal Dashboard API call to return a network only returns combined network IDs, it could be difficult to leverage this endpoint.
Network ID Formats
Combined Network (Locale) ID: L_1234567890
Single Network ID: N_1234567890
Use the Administered Orgs deep link to return the complete Org JSON.
https://api.meraki.com/manage/organization/administered_orgs
The JSON will include an array of networks. Search for the network in question and note the id.
"nL8-zdvc":{
"id":"646829496481144476",
"n":"Sandbox 2 - Ireland - wireless",
"t":"Sandbox-2-Irelan",
"eid":"nL8-zdvc",
"locale_id":"646829496481099051",
"config_template_ng_id":"646829496481121612",
"network_type":"wireless",
"network_tags":" sandbox ",
"time_zone":"America/Los_Angeles",
Construct the new network ID by appending an n_ , for example the above ID would look like N_646829496481144476
Now you can now return a client’s Splash Authorization like this:
$ curl -X GET \
> https://n143.meraki.com/api/v0/networks/N_646829496481144476/clients/aa:bb:cc:f7:ff:08/splashAuthorizationStatus \
> -H 'X-Cisco-Meraki-API-Key: be647eed304654BoGuSd07b7f7ceffffff02de'
{"ssids":{"4":{"isAuthorized":true,"authorizedAt":"2018-05-07 09:39:32 UTC","expiresAt":"2018-05-08 09:39:32 UTC"}}}$
Hope this helps!