A Study to Assess the Effectiveness of BOTOX (Botulinum Toxin Type A) Injections for the Change of Masseter Muscle Prominence in Adult Participants

Inclusion Criteria: * Body mass index \< 30 kg/m\^2 using the calculation: BMI = weight (kg)/height (m)\^2, rounded to the nearest whole number. * Participants meets the following criteria: * Bilateral Grade 4 or Grade 5 MMP (identical grades for left and right side of the face), as determined at screening and at the Day 1 visit by the investigator using the MMPS. * Bilateral Grade 4 or Grade 5 MMP (identical grades for left and right sides of the face), as determined at the Day 1 visit by the participant using the MMPS-P. * MMP grades, as assessed by investigator and participant, do not need to be identical but have to be a minimum of Grade 4 with bilateral symmetry at Day 1. * Participants who responded "A lot" or "Extremely" bothered by the appearance of their MMP as measured by the Bother Impact Assessment for Masseter Muscle Prominence (BIA-MMP) and attain a total score of ≥ 15 on the Lower Facial Shape Questionnaire - Impact Assessment (LFSQ-IA), determined at the Day 1 visit. Exclusion Criteria: * Has current intraoral infection, including infection of the mouth or gums, or facial skin infection requiring medical treatment in the opinion of the investigator. * Has weakness of the masseter, pterygoid, or temporalis muscles due to trauma, facial nerve injury, or other condition that could interfere with normal chewing and jaw clenching, as determined by the investigator. * Excess lower facial fat, jowling, loose or lax skin in lower face, or parotid gland prominence that could interfere with MMPS or MMPS-P grading, as determined by the investigator. * Has medical condition that may put the participant at increased medical risk with exposure to BOTOX, including diagnosed myasthenia gravis, Eaton-Lambert syndrome, amyotrophic lateral sclerosis, or any other condition that might interfere with neuromuscular function. * History of dental or surgical procedure for lower facial shaping or masseter muscle reduction. * History of any soft tissue fillers in the jawline. * Has prior exposure to botulinum toxin of any serotype to any part of the body (not including masseter muscle) within the 3 months prior to Day 1. * History of or current TMJD, or presence of signs/symptoms of possible TMJD in the opinion of the investigator.