{
    "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentParameters.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {
        "virtualNetworkAddressPrefix": {
            "value": [ "10.0.0.0/16" ]
        },
        "virtualNetworkSubnetAddressPrefix": {
            "value": "10.0.0.0/16"
        },
        "agentPoolVmSku": {
            "value": "Standard_D16s_v3"
        },
        "maxpods": {
            "value": 20
        },
        "nodecount": {
            "value": 4
        },
        "mincount": {
            "value": 4
        },
        "maxcount": {
            "value": 10
        },
        "linuxSSHKey": {
            "value": "GEN-SSH-PUB-KEY"
        }
    }
}
{
    "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentParameters.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {
        "virtualNetworkAddressPrefix": {
            "value": [ "10.0.0.0/16" ]
        },
        "virtualNetworkSubnetAddressPrefix": {
            "value": "10.0.0.0/16"
        },
        "agentPoolVmSku": {
            "value": "Standard_D16s_v3"
        },
        "maxpods": {
            "value": 20
        },
        "nodecount": {
            "value": 4
        },
        "mincount": {
            "value": 4
        },
        "maxcount": {
            "value": 10
        },
        "linuxSSHKey": {
            "value": "GEN-SSH-PUB-KEY"
        },
        "adminUsername": {
            "value": "clusterAdmin"
        }
    }
}

Add default value of adminUsername
Add default value of adminUsername