Inclusion Criteria:

          -  Participants should weigh at least 35 kg.

          -  Eastern Cooperative Oncology Group performance status of 0 or 1 and a life expectancy
             of >= 3 months.

          -  Participant have >= 1 lesion accessible for intratumoral injection.

          -  Histologically or cytologically confirmed R/M HNSCC (of the following 4 subsites: oral
             cavity, oropharynx, larynx, and hypopharynx) who previously progressed either during
             or after <= 3 prior treatment regimens administered in the recurrent or metastatic
             setting.

               -  Must have received 1 immunotherapy regimen which included a PD-(L)1 inhibitor.

               -  Must have received platinum-based therapy, or be considered ineligible for
                  platinum-based therapy by the investigator.

        Exclusion Criteria:

          -  Uncontrolled metastases to the central nervous system (CNS).

               -  Participants with brain metastases are eligible provided that evidence of
                  clinical and radiographic stable disease for at least 4 weeks after definitive
                  therapy is given and participants have not used prohibited levels of steroids for
                  at least 4 weeks prior to first dose of the study.

          -  Received prior treatment with OX40 or toll-like receptor (TLR) agonists (excluding
             topical agents).