In March 2019, he was named in Namibia's squad for the 2019 ICC World Cricket League Division Two tournament.[4] Namibia finished in the top four places in the tournament, therefore gaining One Day International (ODI) status.[5] Birkenstock made his ODI debut on 27 April 2019, against Oman, in the tournament's final.[6]
In June 2019, he was one of twenty-five cricketers to be named in Cricket Namibia's Elite Men's Squad ahead of the 2019–20 international season.[10][11] In September 2019, he was named in Namibia's squad for the 2019 ICC T20 World Cup Qualifier tournament in the United Arab Emirates.[12] In September 2021, Birkenstock was named in Namibia's squad for the 2021 ICC Men's T20 World Cup.[13]
Birkenstock's father Rudi Birkenstock is a cricket umpire who has umpired women's international games, and has also played cricket for Namibia's over-50's team. His cousin Uwe Birkenstock has played first-class cricket in South Africa.[14]