Currently the Azure DevTest Lab service does not let you use managed disks during the VM deployment, because it does not support them.
When you have the need for managed disks in an Azure DevTest Lab, you can use the environment capability with Azure Resource Manager templates.
Create an Azure Resource Manager template for a VM deployment with managed disks and upload it to GitHub repository. The next step is to configure the DevTest Lab to use the GitHub repository for the ARM templates.
-> https://azure.microsoft.com/en-us/blog/announcing-azure-devtest-labs-support-for-creating-environment-with-arm-templates/
-> https://blogs.msdn.microsoft.com/devtestlab/2016/11/16/connect-2016-news-for-azure-devtest-labs-azure-resource-manager-template-based-environments-vm-auto-shutdown-and-more/
After that DevTest Lab users are able to deploy VMs with managed disks.
