"The test went really well - we were on the pace pretty quickly and that gave us a boost to give it a crack," he said.
"We had a look at the times from the race season last year and the times were pretty similar. The test driver for Toyota the day before - we were setting similar times to him on the same tyre and similar conditions. We were in the ballpark.
"I had never driven the car before and it went favourably."
Harker joins two other Kiwis - series regular Brendan Leitch and Italian Formula 4 champion Marcus Armstrong - as confirmed entrants. More international drivers will join the list of drivers looking to get valuable seat-time in a respected championship over the northern hemisphere summer.
"It is a big step up from the 86 Championship," Harker said. "The 86 series was great and I learned a huge amount but TRS is on another level. It is an international level with the best drivers from around the world."
And the 22-year-old isn't there to make up numbers or gain experience. He expects to be competitive this season.
"I'm certainly looking to be a front-runner. It is hard to see at the moment not being on track with the other drivers to see where I stack up. If not in the early stages, definitely by the end of the series I hope to be fighting for podiums and even race wins."
Beyond this summer Harker does harbor ambitions to race overseas but nothing is yet confirmed.
"The goal for me would be to put together a budget after this to go to Asia or Australia."