06/03/2014 at 11:00 AM EDT
She thought she was just going for a stroll after a lunch date.
But what Jill Duggar, 22, didn't
know was that her boyfriend, Derick Dillard, had been planning a surprise proposal
that included a serenade by country singer Walker Hayes
, who had written a song about their relationship.
The romantic moment will be featured on the season finale of TLC's hit reality show 19 Kids & Counting
, airing Tuesday (9 p.m. ET).
In a sneak peek of the episode, Jill – the second daughter of Jim Bob, 48, and Michelle, 47 – and Dillard, who were introduced by her father
, listened as musicians hiding in the park came in to join the performance.
"Last minute, I was going over what I was going to say in my head," Dillard says in the clip. "I was trying to play it cool."
But Jill may have suspected something big was about to happen: "I ... am like on the alert, basically, like, 'What's going on?' " she says.
Dillard and Duggar began courting after Jill's sister Jessa, 20, began a courtship with Ben Seewald
, 18. The two are currently planning a wedding – and saving their first kiss for their wedding day.