The Holden outfit said announcements about its 2019 partners would be made in the coming weeks, for the ZBs of Scott Pye and James Courtney.
Adderton noted Boost held discussions with a range of teams, and was even ready to sit out major sponsorship in 2019.
The eventual deal with GRM then "came about fairly quickly" from mid-December.
"As we've said from Day 1, we have a very specific vision in mind for what we want for the Boost brand and the role we want to play in Supercars," he added.
"We were very clear with all of the teams we were talking to – and we were talking to a lot of teams – that we wanted to have a young driver line-up, we wanted to have it called Boost Mobile Racing with our own naming rights and we wanted to have a two-car team.
"We talked to a lot of teams and we were willing to sit this year out if we didn't get the team and the guys we wanted.
"At the last minute, Garry and I had a conversation.
"We made it clear to Garry what we wanted and we said we want young drivers in the car and there were a bunch of young drivers to look at and obviously we'd had a relationship with Richie."
Both Stanaway, 27, and GRM incumbent James Golding, 22, are entering their second Supercars seasons in 2019.
Stanaway endured had a torrid rookie campaign with Tickford Racing, splitting with the Ford squad last month, after impressing during the 2016 and '17 PIRTEK Enduro Cups.
Golding was expected to be partnered by veteran Tander at GRM again before Wednesday's news, which likely ends the latter's full-time driving career.