main.tf

module "subnet" {
  source = "./subnet_module.tf"
}
  
  
resource "azurerm_virtual_machine" "some_name" {
  ...
  depends_on = module.subnet
  subnet_id = module.subnet.id
}

subnet_module.tf

resource "azurerm_subnet" "default" {
  ...
  # https://www.terraform.io/docs/providers/azurerm/r/subnet.html
}

output "id" {
  value = azurerm.subnet.id
 }