This study is for adults who need open surgery to repair a very large abdominal (ventral) hernia. This study tests whether a one-time, image-guided injection of a medicine commonly known as BOTOX® (onabotulinumtoxinA) into the side abdominal muscles 3-7 weeks before surgery helps surgeons close the abdominal wall fully at the end of the operation. Closing the muscle and tissue layers (primary fascial closure) is linked to fewer problems after surgery and better quality of life. Participants will be randomly assigned (like a coin flip) to receive either the BOTOX® medicine or a saltwater (placebo) injection. Neither patients nor the care team will know which one was given. All participants will still have their planned hernia repair in the standard way that we repair patients who are not part of the study. The study will track whether the abdomen can be closed without leaving a gap, and investigators will also look at recovery, complications, time in the ICU or on a ventilator, length of stay, pain, and quality-of-life scores. Most information will be collected during participants' hospital stay, but the investigators will continue to see how participants are doing up to 2 years after surgery. Short phone check-ins will occur before surgery, and after surgery follow-up happens around 30 days, 90 days, 1 year, and 2 years. Possible risks from the injection include temporary muscle weakness, trouble swallowing or breathing, pain or infection at the injection site, and (if CT is used for guidance) a small amount of radiation exposure. Surgery itself carries the usual risks (pain, bleeding, wound problems). Benefits are not guaranteed, but the injection may make closure easier and recovery smoother. About 188 people will take part at Cleveland Clinic.