Miyata will share the #57 Kessel Racing Ferrari 488 GTE for his first race experience of the WEC with Takeshi Kimura and Scott Huffaker, replacing an absent Daniel Serra.
The Super Formula and SUPER GT racer was recruited after Kimura contacted Toyota to enquire about Miyata's availability, with the Japanese manufacturer giving its blessing.
Despite a lack of preparation time, Miyata said he is "really excited" about the chance to make his WEC race debut after attending the previous rounds at Le Mans and Monza in his capacity as a Toyota WEC Challenge driver.
"We only discussed it on Tuesday of this week, while I was driving at Motegi for the SUPER GT test," Miyata told Motorsport.com. "I want to say thank you so much to Toyota and to Kimura-san for this opportunity.
"I came here directly from Motegi. I had to focus on the SUPER GT test, but at lunchtime [on Wednesday] I was already reading the manual and checking some onboards.
"I know Fuji well, but it's my first time in the car and also on Michelin tyres. Also it looks like we could have bad weather on Friday, so I am worried about whether I can run properly during free practice.
"But I think if I can learn how to drive the Ferrari and the tyre in practice, I think I can fight in the race. This opportunity is really good for me and for my career in the future."
Toyota Gazoo Racing Europe technical director Pascal Vasselon said the marque was keen for Miyata to gain experience of the WEC as he is groomed for a potential future Hypercar seat.
"It's a good opportunity for him to get started," said Vasselon. "Toyota is keen to have him driving, especially in a WEC environment, so it was automatically approved.
"It's part of his preparation for GT3, even if it's a GTE car this time, and for the WEC as well, because clearly he is in the frame to join the WEC in the future."
Miyata stressed that his 2024 plans remain undecided, with the WEC schedule currently featuring two clashes with SUPER GT as things stand.
But he said driving in the GTE Am class at Fuji could be a valuable experience if he ends up participating in the WEC next year as part of the LMGT3 category.
"This time I can learn how the WEC race week works, and how to share the track with Hypercars," he said. "If I drive GT3 next year, I have to be careful about that.
"In SUPER GT I am the one overtaking [GT300 cars], but next season maybe I will have to take care of what's coming behind me. I will have to learn how to watch the mirrors!"
