I urge you to tell your partner about the HRT ASAP. If my partner started on HRT without telling me, I would be so hurt that I wasn't included in their journey. It's better to tell the truth and give her time to get used to it than Abe her potentially stumble upon the knowledge that you're taking it without telling her. And as someone who stumbled upon the fact that their partner was trans before their partner could come out to them, I will tell you the chance of them finding out on their own is high.