It's in His Kiss: The 2nd Epilogue (Bridgertons 7.5)

And then she bellowed it.

“I DID IT!!!!”

She looped the necklace around her neck, still clutching the bracelet and ring in her hand.

“I did it, I did it, I did it.” She was singing it now, hopping up and down, almost dancing, almost crying. “I did it!”

“Hyacinth!” It was Gareth, out of breath from taking four flights of stairs two steps at a time.

She looked at him, and she could swear she could feel her eyes shining. “I did it!” She laughed, almost crazily. “I did it!”

For a moment he could do nothing but stare. His face grew slack, and Hyacinth thought he might actually lose his footing.

“I did it,” she said again. “I did it.”

And then he took her hand, took the ring, and slipped it onto her finger. “So you did,” he said, leaning down to kiss her knuckles. “So you did.”

Meanwhile, one floor down…

“Gareth!”

Isabella looked up from the book she was reading, glancing toward the ceiling. Her bedchamber was directly below the nursery, rather in line with the washroom, actually.

“I did it!”

Isabella turned back to her book.

And she smiled.