I am using this along with cloudflare DNS to create a CNAME


data "azurerm_client_config" "current" {}

resource "azurerm_resource_group" "example" {
  name     = "example-resources"
  location = "West Europe"
}

resource "azurerm_web_pubsub_service" "example" {
  name                = "example-webpubsub"
  location            = azurerm_resource_group.test.location
  resource_group_name = azurerm_resource_group.test.name

  sku {
    name     = "Premium_P1"
    capacity = 1
  }

  identity {
    type = "SystemAssigned"
  }
}

resource "azurerm_key_vault" "example" {
  name                = "examplekeyvault"
  location            = azurerm_resource_group.example.location
  resource_group_name = azurerm_resource_group.example.name
  tenant_id           = data.azurerm_client_config.current.tenant_id
  sku_name            = "premium"

  access_policy {
    tenant_id = data.azurerm_client_config.current.tenant_id
    object_id = data.azurerm_client_config.current.object_id

    certificate_permissions = [
      "Create",
      "Get",
      "List",
    ]

    secret_permissions = [
      "Get",
      "List",
    ]
  }

  access_policy {
    tenant_id = data.azurerm_client_config.current.tenant_id
    object_id = azurerm_web_pubsub_service.test.identity[0].principal_id

    certificate_permissions = [
      "Create",
      "Get",
      "List",
    ]

    secret_permissions = [
      "Get",
      "List",
    ]
  }
}

resource "azurerm_key_vault_certificate" "example" {
  name         = "imported-cert"
  key_vault_id = azurerm_key_vault.example.id

  certificate {
    contents = filebase64("certificate-to-import.pfx")
    password = ""
  }
}

resource "azurerm_web_pubsub_custom_certificate" "test" {
  name                  = "example-cert"
  web_pubsub_id         = azurerm_web_pubsub_service.example.id
  custom_certificate_id = azurerm_key_vault_certificate.example.id

  depends_on = [azurerm_key_vault_access_policy.example]
}