You can let us know about your pre-payment certificate while registering with Phlo or add a new certificate after signing up through the Profile page, under the exemptions section of the app.
About pre-payment certificates
Pre-payment certificates can significantly reduce the cost of medication. You can read more about how they work here: Buy an NHS Prescription Prepayment Certificate.
If you would like to purchase a pre-payment certificate you can do so by visiting this link.