Thank you for your message! If you are taking the EuroStar and you are leaving the EU then there will be a place to drop off your Tax VAT refund at the train station. If you are not leaving the EU which you indicate you are not, then you will drop off your tax refund either at the last train station you are out or the last airport you are out before you leave the EU. I’ve written an article that outlines this entire process that you might find helpful! https://petiteinparis.com/vat-tax-refund-process-in-paris-france/
I hope you have a great time and that answers your question!