A force of $-F\hat{k}$ acts on $O$, the origin of the coordinate system. The torque about the point $(1,-1,0)$ is:

Torque $\vec{\tau }=\vec{r}\times \vec{F}$.Here, $\vec{r}$ is the vector from the point of rotation to the point of application of force.Point of rotation $P=(1,-1,0)$. Point of application $O=(0,0,0)$.$\vec{r}=\overrightarrow{r_O}-\overrightarrow{r_P}=(0-1)\hat{i}+(0-(-1))\hat{j}+(0-0)\hat{k}=-\hat{i}+\hat{j}$.$\vec{F}=-F\hat{k}$.$\vec{\tau }=(-\hat{i}+\hat{j})\times (-F\hat{k})=F(\hat{i}\times \hat{k})-F(\hat{j}\times \hat{k})$ Using cross product rules ($\hat{i}\times \hat{k}=-\hat{j}$, $\hat{j}\times \hat{k}=\hat{i}$):$\vec{\tau }=F(-\hat{j})-F(\hat{i})=-F(\hat{i}+\hat{j})$.