Choose Fixed Height and continue with the next step.

or

Choose Connect Shapes. To set the form's height to that of existing shapes or forms to connect to, select those shape(s) and/or form(s). Confirm the selection with a data point. Reset to continue.

or

Choose Connect 3pts and then place three data points to define the plane in which the top face of the form is located.

or

Choose Connect 2pts/View and then place two data points to define an imaginary line. Enter a data point in the view that the top face's plane is perpendicular to.