• Participants must have a confirmed histological diagnosis of cutaneous melanoma or melanoma of unknown primary.

• Participants must have stage IV or advanced unresectable disease.

• No prior ICB treatment for advanced unresectable or metastatic disease. Participants may have received adjuvant or neoadjuvant ICB if last dose was given ≥ 6 months prior to enrollment

• Prior targeted therapy with BRAF/MEK inhibition in the adjuvant or advanced / metastatic setting is permitted if at least 2 weeks have elapsed between the last dose and study enrollment. Participants must have recovered to ≤ grade 1 from all toxicity related to BRAF/MEK inhibition

• Prior radiation therapy is permitted if at least 7 days have elapsed between the last fraction and study enrollment. Participants must have recovered to ≤ grade 1 from all toxicity related to prior radiotherapy.

• Previous major surgery is permitted provided that surgery occurred ≥ 14 days prior to participant enrollment and that wound healing has occurred.

• Participants must have measurable disease as per RECIST 1.1/ iRECIST.

• Participants must be at least 18 years of age.

• Participants must have an ECOG performance status of 0, 1, or 2.

• The participant's standard-of-care ICB regimen must be selected prior to enrollment and must stay the same, regardless of arm assignment, post-enrollment

• Participants must demonstrate adequate organ function Participants must be able to ingest capsules.

• Participants must consent to provision of samples of blood and stool for correlative marker analysis.

• Participants must consent to provision of, and investigator must agree to submit, a representative archival formalin fixed paraffin block of tumour tissue for correlative analyses when tumour tissue is available.

• Participants must have access to provincially-funded standard-of-care ICB treatment.

• Participant consent must be appropriately obtained in accordance with applicable local and regulatory requirements. Each participant must sign a consent form prior to enrollment in the trial to document their willingness to participate.

• Participants must be accessible for treatment and follow-up. Investigators must assure themselves the participants randomized on this trial will be available for complete documentation of the treatment, adverse events, and follow-up.

• Protocol ICB treatment must begin within 14 calendar days after participant enrollment.

• Participants of childbearing potential must have agreed to use a highly effective contraceptive method.