I thnk you can do that in the properties of the extension (right click on the extension)
Below the field which says "Dimension" there is "Formula".
I guess you can add there your Dimension (click on green plus)
and set the condition. (click on the button on the right side)
I hope that helps.
Are you talking about this field ?
If so this field is not affecting the extension. The checkbox (Filter By Expression) is always unchecked (after i re open the property panel).
The main goal is to gain performance with extension. So i would like a conditional dimension to request lesser data from QV with the property panel.