Extension Study to Evaluate NBI-827104 in Pediatric Participants With Epileptic Encephalopathy With Continuous Spike-and-Wave During Sleep

Key Inclusion Criteria: For participants who enroll directly following the completion of the Study NBI-827104-CSWS2010: * Completed 12 weeks of treatment in Study NBI-827104-CSWS2010. For Participants Who Do Not Enroll Directly from or Did Not Participate in Study NBI-827104-CSWS2010: * Have diagnosis of EECSWS confirmed by the Diagnosis Confirmation Panel (DCP). Key Exclusion Criteria: For participants who enroll directly following the completion of the Study NBI-827104-CSWS2010: * Have developed any other disorder for which the treatment takes priority over treatment of EECSWS or is likely to interfere with study treatment or impair treatment compliance. For Participants Who Do Not Enroll Directly from or Did Not Participate in Study NBI-827104-CSWS2010: * Body weight \<15 kg at Day 1. * Clinically relevant findings related to cardiovascular or laboratory parameters at screening as determined by the investigator. * Presence of relevant neurological disorders other than EECSWS and its underlying conditions as judged by the investigator. Symptomatic conditions underlying EECSWS (for example, neonatal strokes) have to be stable for at least 1 year prior to screening. * Planned surgical intervention related to structural abnormalities of the brain from screening through the Week 6 Visit. * Used any active investigational drug other than NBI-827104 in the context of a clinical study within 30 days or 5 half-lives (whichever is longer) before screening or plans to use such an investigational drug (other than NBI-827104) during the study. * Have developed any other disorder for which the treatment takes priority over treatment of EECSWS or is likely to interfere with study treatment or impair treatment compliance.