JWST NIRISS imaging is only offered as a coordinated parallel mode. Dither patterns are therefore selected in the APT interface of the prime science instrument.
Parallel NIRISS imaging uses primary instrument dithers
The NIRISS imaging mode will not be available as a prime observing mode for GO programs, but it will be allowed as a "parallel" mode in science parallels. As such, dithering will be selected through the APT template of the prime instrument mode. Note that direct imaging with NIRISS will be severely affected by undersampling at λ ≤ 4μm. Thus, dithering is important for this mode.