@CA Gary Mastin Thanks for reaching out. To create an APIM instance in stv1 Platform you need to create APIM with VNET not specifying a management public IP creates the resource in stv1.
You can use below powershell command to create APIM in STV1
$virtualNetwork = New-AzApiManagementVirtualNetwork -SubnetResourceId "/subscriptions/a8ff56dc-3bc7-4174-b1e8-3726ab15d0e2/resourceGroups/ContosoGroup/providers/Microsoft.Network/virtualNetworks/westUsVirtualNetwork/subnets/backendSubnet"
New-AzApiManagement -ResourceGroupName "ContosoGroup" -Location "West US" -Name "ContosoApi" -Organization Contoso -AdminEmail ******@contoso.com -VirtualNetwork $virtualNetwork -VpnType "External" -Sku "Premium"
do let me know incase of further queries, I would be happy to assist you.