DA-20 student pilot reported an NMAC event while conducting a check ride and transmitting on the wrong frequency. Reporter took evasive action to avoid the other aircraft.

Narrative

I was a student pilot conducting a check ride. I didn't realize I had fat fingered the radio frequency for the local practice area. I was making applicable radio calls stating my intentions with 1 - 2 other airplanes in the area that were never a factor throughout the duration of the flight. I conducted clearing turns prior to every maneuver and checked ADS-B to ensure I was deconflicted from other aircraft. After my maneuvers concluded; I checked the ATIS inbound to ZZZ. Once I had the current ATIS; I made a final call to what I thought was the practice area frequency - XXX.XX. However; I was transmitting on XXX.X when I had noticed another Cessna below and behind me within approximately 500 ft. vertical separation tracking towards ZZZ also. I banked right and began climbing to deconflict myself. I contacted ZZZ Tower on YYY.Y to request landing clearance when the other pilot asked why I wasn't on communications and they had been trying to contact me. That was when I realized my mistake that I had not clicked the radio to the correct frequency. I will ensure during future flights to double and triple-check that my frequencies are keyed correctly and conduct a radio check with other airplanes in the practice area. I made the assumption that my calls were being made and the few other planes in the vicinity were well enough deconflicted that maybe they weren't making calls. I understand the importance of making clear and concise radio calls on the correct frequency; to ensure safety and deconfliction from others. I will be more vigilant in the future when keying radios; and not let the task at hand skew my focus and judgment to mis-key and assume my calls were on the right frequency.