How to add arguments to karma-mocha?

I have set unit testing for a TypeScript project. From command line this works fine:

mocha --compilers ts:typescript-require ./source/**/*.spec.ts

Now, I have added mocha and chai to my karma file like so:

...
frameworks: ['mocha', 'chai'],
...

How do I include the argument compilers ts:typescript-require to mocha using the karma.conf.js file using this section:

...
client: {
  mocha: {
    // I assume it goes here?
  }
},
...

An example would be greatly appreciated!