24 Weeks Double-blind Randomized Placebo-controlled Trial to Evaluate Efficacy, PK, Safety of LOU064 in Adolescents (12 - <18) With CSU and Inadequate Response to H1-antihistamine Followed by Optional 3 Years Open-label Extension and an Optional 3 Years Safety Long-term Treatment-free Follow-up

Key Inclusion Criteria: * Male and female adolescent participants aged \>= 12 to \< 18 years of age at the time of signing the informed consent * CSU duration for \>= 6 months prior to screening (defined as the onset of CSU determined by the investigator based on all available supporting documentation) * Diagnosis of CSU inadequately controlled by second-generation H1-AH at the time of randomization defined as: * The presence of itch and hives for ≥ 6 consecutive weeks prior to screening despite the use of second-generation H1-AH during this time period according to local treatment guidelines * UAS7 score (range 0 - 42) \>= 16, ISS7 score (range 0 - 21) \>= 6 and HSS7 score (range 0 - 21) \>= 6 during the 7 days prior to randomization (Day 1) * Documentation of hives within three months before randomization (either at screening and/or at randomization; or documented in the participants' medical history) Key Exclusion criteria: * Previous use of remibrutinib or other BTK inhibitors * Significant bleeding risk or coagulation disorders * History of gastrointestinal bleeding * Requirement for anti-platelet medication, except for acetylsalicylic acid up to 100 mg/d or clopidogrel up to 75 mg/d. The use of dual anti-platelet therapy (e.g., acetylsalicylic acid + clopidogrel) is prohibited * History or current hepatic disease * Evidence of clinically significant cardiovascular, neurological, psychiatric, pulmonary, renal, hepatic, endocrine, metabolic, hematological disorders, ga…