How can I add trial period to my Ghost members subscriptions

I got feedback that some members would like to have trial periodonmy premium subscrition in my ghost blog: https://uklad.vc

How can I implement it?
Is there some JS code where I can modify Stripe API call (https://stripe.com/docs/api/subscriptions/create) so that I add 3d-trial

 "trial_period_days": null, // change to 3
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