task-fMRI to determine TMS site (online stimulation with that same task)?
This is not a circular question. In fact, using fMRI to create targets is useful to determine TMS targets outside of the motor cortex or visual cortex. See the study here that used fMRI during a task to generate TMS targets, and then applied theta burst stimulation to those targets: https://www.pnas.org/content/113/21/6059

BOLD contrasts are very useful when using TMS to target sites outside of motor cortex, visual cortex and frontal eye field, where TMS can be validated functionally using motor evoked potentials (MEPs), phosphenes or saccadic eye movements, respectively. Brainsight TMS Navigation enables BOLD contrasts (or peak MNI co-ordinates derived from BOLD contrasts) to be entered as targets for TMS.