To enable a Free Trial period for your Android subscriptions:
Open your Google Play Developer Account
Select your Android App
In the next screen, click on In-App Product section in the left-sidebar menu.
In the next screen, select the Subscription you want to add the Free Trial to.
Once open, scroll down Free Trial Period field and allocate a number of days for the free trial. Then click on Submit Update in the top-right corner to save changes