How do I pass my own number value for Trial Period to Paypal?

I am using the Paypal Checkout Add-On and have it all configured apart from one specific thing I would like to do… I am using the Trail Period feature but instead of setting the number of days in the admin panel, I would like to get the number value dynamically.

I know how to generate that number OK, but I can’t work out how to submit it to Paypal ahead of the one stored in my settings. I wondered if it was something to do with the “maybe_enable_trial” function and how I could do this?