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, gastrointestinal disease or immunodeficiency that, in the investigator's opinion, would compromise the safety of the participant, interfere with the interpretation of the study results or otherwise preclude participation or protocol adherence of the participant * History of hypersensitivity to any of the study drugs or its excipients or to drugs of similar chemical classes * Participants having a clearly defined predominant or sole trigger of their chronic urticaria (chronic inducible urticaria) including urticaria factitia (symptomatic dermographism), cold-, heat-, solar-, pressure-, delayed pressure-, aquagenic-, cholinergic-, or contact-urticaria * Other diseases with symptoms of urticaria or angioedema, including but not limited to urticaria vasculitis, urticaria pigmentosa, erythema multiforme, mastocytosis, hereditary angioedema, or drug-induced urticaria * Any other skin disease associated with chronic itching that might influence in the investigator's opinion the study evaluations and results, e.g., atopic dermatitis, bullous pemphigoid, dermatitis herpetiformis, senile pruritus or psoriasis Other protocol-defined inclusion/exclusion criteria may apply.