As far as I know, Blender doesn't have that feature. And settings the percentage doesn't work, because it is limited to whole numbers.
Here is a workaround using math.
Lets say the resolution is 1920 x 1080 and you want to scale the width down to 1000, then the following will keep the aspect ratio.
width / previousWidth * newWidth = newWidth
height / previousWidth * newWidth = newHeight
The same applies the other way round.
Since the current width is already input in the resolutions X field, we can go into that field and add the number for previousWidth x newWidth. When exiting the field the value will be calculated. Repeat the same in the resolutions Y field with the same data.
This is how it looks in action.
Since pixels are whole numbers, the exact proportions can not always be retained.