CODE EXAMPLE FOR CSS

how do I make secret keys available in gatsby-config from Netlify

{
    resolve: 'gatsby-source-mautic',
    options: {
        baseUrl:  process.env.MY_MAUTIC_BASE_URL,
        auth: 'basic', [optional]
        publicKey: [your username],
        secretKey: process.env.MY_SECRET_KEY
    }
}