We have shown that coupling a textbook ground state DA reaction to a quantized cavity photon allows for strong isomer selectivity, which is not observed outside the cavity under standard reaction conditions. Our results show that the cavity induces selectivity toward the endo isomer, even for moderate coupling strength as well as for random molecular orientations. In addition, we have shown that the pQED-TDDFT method qualitatively agrees with the high-level scQED-CCSD approach and with errors between the two approaches less than 3 kcal/mol, in agreement with reported errors between DFT and CCSD methodologies outside the cavity. This work, thus, provides an important and necessary benchmark with the highlevel self-consistent QED coupled cluster approach which is commonly used to explore ab initio molecular polaritons.