No. On the EOS R6, EF-S lenses automatically enable 1.6x crop mode, and Canon’s documentation says no other option is available.

The reason is optical as well as electronic: EF-S lenses project a smaller image circle meant for APS-C sensors, not the R6’s full-frame sensor. Even if you could bypass the camera’s detection, the lens still would not properly cover the full sensor, so you’d get very heavy vignetting and likely poor corner quality, distortion, and softness.

A workaround may be possible only by preventing the camera from recognizing the lens, such as with certain third-party adapters or by blocking lens contacts. But that would leave you with major limitations like manual focus, shooting wide open, and needing to enable release without lens. Image quality would still usually be poor, though some crop lenses may cover more at certain focal lengths.

So in normal use with Canon’s adapter, you cannot force full-frame capture with an EF-S lens on the R6.