The reserve driver, who lost his race seat last season, was called up after Red Bull regular Isack Hadjar injured his wrist in training.
New Zealander Liam Lawson has moved up from Racing Bulls to replace Hadjar, with Tsunoda taking the slot alongside rookie Arvid Lindblad.
“I was with my manager, and also other friends, in Greece spending time together, and I was on the beach," Tsunoda told reporters on Thursday of how he heard the news.
"I was still training and everything. At the moment I received a call from my manager...(I was) pretty relaxed, watching the nice beautiful ocean, and I just suddenly got a call, ‘OK, you have to go to the UK’."
Tsunoda said he was trying to stay calm after being handed the opportunity he had been waiting for.
"I was dying to have this kind of moment... I don’t really mind which car I’m driving to be honest, I’m already grateful enough to have a seat," said the Japanese, who might have hoped to replace Hadjar directly as the reserve for both teams.
"Both teams are performing well, especially Racing Bulls this year."
The Dutch weekend at Zandvoort is the first grand prix since the August break and will follow the sprint format, another challenge for Tsunoda who has also to come to terms with the new generation of car in a very different engine era.
