[5] Later that year, on 31 August, Postema made his professional debut in a 3–1 Eredivisie loss to Heracles Almelo, coming on as a substitute for Amir Absalem.

[11] Postema returned to Groningen for the 2021–22 season, and on 27 October 2021, he stood out during a KNVB Cup match against Helmond Sport by scoring two goals and helping his team win 4–0.

[12] On 12 March 2022, he came off the bench in the 78th minute and made an impressive impact, scoring his first two goals for Groningen and helping the team secure a thrilling 4–3 victory over NEC.

[18] Postema scored six goals and provided two assists in 32 competitive appearances for Roda and returned to recently relegated Groningen at the end of the season.

[19] After returning to Groningen following his loan spell at Roda JC, Postema immediately established himself as a starter in head coach Dick Lukkien's side.

[3] Upon joining Roda JC on loan in 2022, head coach Jurgen Streppel described Postema as a "technically skilled player with a keen sense for scoring goals.