A Study to Investigate How Effective, Safe and Tolerable the Drug NBI-921352 is When Used With Anti-seizure Medications in Adults With Focal Onset Seizures

Key Inclusion Criteria: 1. Capable of providing consent and has completed the written informed consent. 2. Male or female, 18 to 65 years of age, inclusive, with a body mass index (BMI) \< 40 kg/m\^2. 3. Diagnosis of focal onset epilepsy according to the International League Against Epilepsy (ILAE) Classification of Epilepsy (2017) at least 18 months before screening. 4. History of uncontrolled seizures despite adequate treatment with at least 1 anti-seizure medication (ASM) for at least 18 months prior to screening. 5. Treatment with at least 1 but not more than 4 ASMs for at least 1 month before screening, during the baseline seizure diary data collection, and throughout the duration of the study. 6. Be able to keep accurate seizure diaries. 7. Documented seizure frequency in the baseline seizure diary of ≥8 countable focal seizures during the 8-week seizure baseline period. Key Exclusion Criteria: 1. History of epilepsy with only nonmotor seizures without an observable component, psychogenic nonepileptic seizures, or primary generalized seizures. 2. Presence or previous history of developmental and/or epileptic encephalopathy. 3. Presence of seizure types other than FOS. 4. Status epilepticus within the last 12 months before enrollment. 5. Any suicidal behavior or suicidal ideation of type 4 (active suicidal ideation with some intent to act, without specific plan) or type 5 (active suicidal ideation with specific plan and intent) based on the C-SSRS in the 2 years before screening, a history of suicide attempt in the last 2 years, or more than 1 lifetime suicide attempt. 6. Multiple drug allergies or a severe drug reaction to an ASM(s), including dermatological (eg, Stevens-Johnson syndrome), hematological, or organ toxicity reactions. 7. An implanted responsive neurostimulator system (RNS).