A Study to Determine the Safety, Tolerability, Pharmacokinetics, and Preliminary Efficacy of ABBV-368 Plus Tilsotolimod and Other Therapy Combinations in Participants With Recurrent/Metastatic Head and Neck Squamous Cell Carcinoma

* 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.

* 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).