This is what will happen.
At 10km to go on your free 25km a countdown will appear on the screen.
When the countdown gets to zero a popup will appear saying your 25km has expired and will give you two options. One is to enter payment details and the other is NOT NOW.
Click on the NOT NOW option and the popup will go away and you can continue riding (beyond the free 25km).
After ending your ride the next time you start Zwift there will be a warning that your 25km has expired and there will be a “Restore Purchase” button (which should work, I haven’t tested that particular bit).
So, to get to the “Restore Purchase” button you have to use all of the 25km trial first. Only after the free trial kms have been used will the “Restore Purchase” button appear on the home screen.
Which is not how Zwift used to work, the “Restore Purchase” button was always on the home screen if you had dropped back to the free trial.