How to tell VSTS to sort usings with System namespaces at the top

Go to Tools ⇨ Options... ⇨ Text Editor ⇨ C# ⇨ Advanced.

On the group Organize Usings make sure that the option Place 'System' directives first when sort usings are checked.

When this setting is checked, sorting using statements by right-clicking on them and using the Organize Usings feature, places System namespaces alphabetically along all other namespace imports.