Sounds like you might need to use one API call to list the networks in an organization, and then run a loop to make a 2nd API call for each returned network to list the VLANs for each network but only process the VLAN 1 info. You should be able to grab your organization id by going to https://dashboard.meraki.com/api/v0/organizations in a browser where you're logged into Dashboard. Once you have your Org ID, you could use the GET https://dashboard.meraki.com/api/v0/organizations/{{organizationid}}/networks and it would return a list of each network in that Org, from there you could gather the network names. Then for each network that got returned, there would be a unique networkId and for each network you could use GET https://dashboard.meraki.com/api/v0/networks/{{networkId}}/vlans which should return a list of all VLANs for each network, and you could just process the single VLAN with "id":1 for VLAN 1 and skip the rest. The returned information includes the subnet as well. Hope that helps a little but I'd suggest opening a case with Meraki Support to assist with getting started.
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