927TiP SKYSCRAPER-09: A phase II, randomised, double-blinded study of atezolizumab (Atezo) + tiragolumab (Tira) and atezo + placebo as first-line (1L) therapy for recurrent/metastatic (R/M) PD-L1+ squamous cell carcinoma of the head and neck (SCCHN)