Additional Study Information: The purpose of this study is to compare the effects of pembrolizumab simultaneously with the Optune device and maintenance temozolomide(TMZ) versus placebo simultaneously with Optune and maintenance TMZ for the study treatment of newly diagnosed GBM. In this study, a placebo is an inactive substance that looks the same as and is given the same way as pembrolizumab. The effects of pembrolizumab are compared to the effects of the placebo. The current United States Food and Drug Administration (FDA) approved treatment for newly diagnosed GBM includes the addition of Optune to the treatment with maintenance temozolomide, following the completion of chemoradiation therapy (radiotherapy (RT) + TMZ). Pembrolizumab is being studied as an immunotherapy drug, to see if it can work with your immune system to help fight cancer. The Optune device generates Tumor Treating Fields (TTFields), which are intended to destroy brain cancer cells. Laboratory studies suggest TTFields' potential to slow tumor growth, with minimal observed side effects in patients with recurrent and newly diagnosed GBM. Pembrolizumab is an experimental drug, which means the FDA has not approved Pembrolizumab for the treatment of GBM. Similarly, the combination of pembrolizumab with Optune and temozolomide (TMZ)has not been approved for the treatment of GBM therefore it is experimental. Optune has been approved for the treatment of recurrent and newly diagnosed GBM by the FDA showing Optune meets the required safety and quality standards.