How to use environment variable in index.html for Angular 6

You should create copy of index.html and name it index.someenv.html.
Then in your angular.json in environment configuration setup file replacement:

"fileReplacements": [
    {
        "replace": "src/index.html",
        "with": "src/index.someenv.html"
    }
]

The angular cli will replace these files when you run your build