I assume that you should be using the Microsoft.Common.TextBox UI element in your createUiDefinition.json.
I have tried to reproduce a green check by creating a simple createUiDefinition.json as below with a Microsoft.Common.TextBox UI element as shown below.
{
"$schema": "https://schema.management.azure.com/schemas/0.1.2-preview/CreateUIDefinition.MultiVm.json",
"handler": "Microsoft.Compute.MultiVm",
"version": "0.1.2-preview",
"parameters": {
"basics": [
{
"name": "textBoxA",
"type": "Microsoft.Common.TextBox",
"label": "VM Name",
"defaultValue": "",
"toolTip": "Please enter a VM name",
"constraints": {
"required": true
},
"visible": true
}
],
"steps": [],
"outputs": {}
}
}
I am able to reproduce the green check beside the VM Name textbox as shown below:
However, this green check DOES NOT imply the VM Name is Available.
This is because based on my testing, even if I use an existing VM Name in the same subscription, it is still showing the green check.
Based on the official documented constraints that are supported by the Microsoft.Common.TextBox UI element, it DOES NOT VALIDATE Name Availability.
Hope this helps!