In air traffic management, integrating new digital solutions is crucial to address growing complexities, increased traffic, and achieve higher operational efficiency for sustainability. As a solution to these problems, the Digital Interactive Reliable Controller (DIRC) is being developed. DIRC is a digital agent designed to work collaboratively with human air traffic controllers (ATCOs) in a Human Autonomy Teaming (HAT) environment. For such collaboration to be successful, a well-designed interface between the human ATCO and the digital partner is essential. To address this need, HI-DIRC (Human Interface to DIRC) has been developed to foster a functional relationship between the human ATCO and DIRC. In order to take into account the expectations and experiences of future users, we applied human-centered design outlined in ISO 9241-210 for interactive systems. The design and development of HI-DIRC was accompanied by a series of workshops in which ATCOs, technical experts, designers and developers came together to share their requirements for the new functions and decide for design solutions. This paper presents the results of the latest collaborative user workshop with active ATCOs of DFS Deutsche Flugsicherung GmbH and Austro Control GmbH, evaluating a functional software prototype of HI-DIRC. This workshop served as the first evaluation of a functional software prototype of HI-DIRC after the initial design phase, emphasizing the user interaction while working with a live system. The overall usability of the functional prototype was positively received, with improvements noted in this most recent workshop compared to the previous workshops, as participants found the tool increasingly supportive and easy to use, highlighting the positive impact of the human-centered design workflow.