How to setup a free trial in Shopify Rails app

[Keemtaker]

I will like to setup a free trial in my shopify rails app. I am using the official Shopify Rails app gem https://github.com/Shopify/shopify_app. The config for billing looks like this

config.billing = ShopifyApp::BillingConfiguration.new(
    charge_name: "Apps",
    amount: 6.99,
    interval: ShopifyApp::BillingConfiguration::INTERVAL_EVERY_30_DAYS,
    currency_code: "USD", # Only supports USD for now
  )

How do I add an option for free trial ? I have had a look at this documentation https://shopify.dev/apps/billing/purchase-adjustments/free-trials but I dont think the api for a free trial is included in the config of the shopify rails gem