Sample createUiDefinition.json file that shows how to provide an experience to select an existing subnet

createUiDefinition-existing-subnet.json

{
	"$schema": "https://schema.management.azure.com/schemas/0.1.2-preview/CreateUIDefinition.MultiVm.json#",
	"handler": "Microsoft.Azure.CreateUIDef",
	"version": "0.1.2-preview",
	"parameters": {
		"basics": [
			{
				"name": "vnetSelector",
				"type": "Microsoft.Solutions.ResourceSelector",
				"label": "Virtual Network",
				"resourceType": "Microsoft.Network/virtualNetworks",
				"options": {
					"filter": {
						"subscription": "onBasics",
						"location": "onBasics"
					}
				}
			},
			{
				"name": "subnets",
				"type": "Microsoft.Solutions.ArmApiControl",
				"request": {
					"method": "GET",
					"path": "[concat(subscription().id, '/resourceGroups/', last(take(split(basics('vnetSelector').id, '/'), 5)), '/providers/Microsoft.Network/virtualNetworks/', basics('vnetSelector').name,'/subnets?api-version=2022-01-01')]"
				}
			},
			{
				"name": "subnetList",
				"type": "Microsoft.Common.DropDown",
				"label": "Subnet",
				"toolTip": "",
				"filter": true,
				"constraints": {
					"allowedValues": "[map(basics('subnets').value, (item) => parse(concat('{\"label\":\"', item.name, '\",\"value\":\"', item.id, '\"}')))]",
					"required": true
				},
				"visible": true
			}
		],
		"steps": [
		],
		"outputs": {
			"location": "[location()]",
			"vnetResourceGroup": "[last(take(split(basics('vnetSelector').id, '/'), 5))]",
			"vnetName": "[basics('vnetSelector').name]",
			"subnetResourceId": "[basics('subnetList')]"
		}
	}
}