To integrate your Flexbooker account with Stripe, we would need both Private and Publishable keys from your Stripe account, here are the steps on how to get that:
https://support.stripe.com/questions/locate-api-keys-in-the-dashboard
Users with Administrator permissions can access a Stripe account’s API keys by navigating to the Developers section of the Stripe dashboard and clicking on API Keys.
Once you have these information you can enter the credentials from the Set up Page then choose the "Payment Settings" label as shown below:
Make sure to click on "Save Settings" to save any changes made.