A force of $-\mathrm{F}\hat{\mathrm{k}}$ acts on origin, then the torque about the point $(1,-1)$ is :

$\overrightarrow{\mathrm{\tau }}=\overrightarrow{\mathrm{r}}\times \overrightarrow{\mathrm{F}}$ Force acts on origin so $\overrightarrow{r}$ is position vector of origin wrt. $(1,-1)$ so

$\overrightarrow{\mathrm{r}}=-\hat{i}+\hat{j},\overrightarrow{\mathrm{F}}=-\mathrm{F}\widehat{\mathrm{k}}$

$\overrightarrow{\mathrm{\tau }}=(-\hat{i}+\hat{j})\times (-\mathrm{F}\hat{\mathrm{k}})=-\mathrm{F}\hat{\mathrm{i}}+\mathrm{F}\hat{\mathrm{j}}$