Parity-time symmetry has great potential for mode selection in multimode resonators. However, in a PT-symmetry system with saturable absorption mechanisms, the random background noise can initiate single-mode oscillation at any of the maxima within the gain spectrum, i.e., potential PT frequencies. Such randomness impedes the acquisition of high-quality signals at desired frequencies. Here, we propose a method to obtain deterministic PT-symmetry single-mode oscillation in a filterless multimode resonator through one-shot injection. With this technique, we change the system's gain spectrum and enhance the gain discrepancy. Utilizing the frequency domain saturable absorption of the resonator, oscillation at desired mode can maintain its frequency after the withdrawal of the injection signal. To validate the concept, we established a polarization-division multiplexed dual-loop optoelectronic oscillator (OEO) with a 1-km long cavity operated under PT-symmetry conditions. By one-shot injecting the PT-OEO, we effectively eliminate the randomness arising from the relatively flat gain spectrum, facilitating oscillation at any desired potential PT frequencies from 1.8 to 9 GHz without requiring elaborate frequency tuning structures. Moreover, the one-shot injection technique produces ultra-low phase noise performance, achieving a remarkable -158.6 dBc/Hz@10 kHz. This performance level stands in close comparison with the best phase noise values recorded for OEOs.