Nuclear Schiff Moment of the Fluorine Isotope <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mmultiscripts> <mml:mrow> <mml:mi mathvariant="normal">F</mml:mi> </mml:mrow> <mml:mprescripts/> <mml:none/> <mml

Nuclear Schiff moments (NSMs) are sensitive probes for physics beyond the Standard Model of particle physics, signaling violations of time-reversal and parity-inversion symmetries in atomic nuclei. In this Letter, we report the first-ever calculation of a NSM in a nuclear ab initio framework, employing the no-core shell model to study the fluorine isotope $^{19}$F. We further perform quantum-chemistry calculations to evaluate the sensitivity of the hafnium monofluoride cation, HfF$^+$, to the NSM of $^{19}$F. Combined with recent high-precision measurements of the molecular electric dipole moment of HfF$^+$, our results enable the first experimental bound on the NSM of $^{19}$F. Although the resulting bounds on the pion-nucleon-nucleon ($π$NN) coupling constants are not yet the most stringent, this work establishes the foundation for constraining $π$NN interactions using nuclear ab initio methods.