The post, from the couple's SussexRoyal account, continued: "Happy Birthday to my wonderful wife".
Speaking to People, Cox revealed that she had no idea that she had been selected by the Duchess of Sussex until she got a surprise phone call. "Happy Birthday to my fantastic wife". Thank you for joining me on this adventure!
For the last couple months, Meghan Markle and Prince Harry have changed the accounts they're following each month to highlight charities doing work around a certain cause (mental health activists and organizations for May's Mental Health Awareness Month, for example).
A source shared: "Meghan is going to become a published author and is very excited about it".
This isn't the first time Samantha has spoken out against her half-sister, previously slamming the duchess for her treatment of their father, Thomas Markle, who Meghan has been estranged from for years.
The first stop was Australia and all eyes were on Meghan because it had just been announced that the Duchess was pregnant. Family members, the royal family, the palace, even palace photographers - EVERYBODY was keen to wish Meghan a very happy 38th birthday here!
Kensington Palace shared a photo of Meghan walking arm in arm with her husband alongside William and Kate, and the Prince of Wales. Happy birthday Meghan!" while another commented: "Harry, I'm so glad that Meghan joined you on this adventure too.
A royal insider told The Sun's Fabulous Digital: "This year there will be a birthday gift from Archie and the odds are on a sepia or black and white photograph of her precious son in a solid silver frame from Aspreys".
The picture was of Meghan and Harry hand in hand and the message read: "Happy Birthday to The Duchess of Sussex".
Not stopping there, the stylist also added a photo of Meghan to her Instagram story, writing: "Happy Birthday". It's unclear why she's reportedly not on the guest list, but it could have something to do with Markle's decision to keep things lowkey. She has two children in her marriage.
"It is testament to Meghan that she has been given this invite".
She says the book will include details of her "home life".
"I loved that - boy or girl, whatever it is, we hope that's the case".