[Affinity Photo: Drag & crop image to pixel dimensions]

A simple problem: I have a photo, and I want it cropped to become Facebook's cover photo (or a featured image on my blog, I have many presets). I have the exact pixel dimensions: 820x340 and want to crop it from a bigger image. But I want to actually draw the area I want cropped and scaled.

 

In Photoshop it works like this:

• you select pixel dimensions
• you drag a rectangle on the image, it has a fixed aspect ratio, but can be any size
• double clicking then crops the image and scales it to the exact pixel dimensions.

I can't figure it out in Affinity Photo. I can have a preset with exact pixel dimensions, but when I scale the area, the pixel dimensions change as well. I want the program to apply the pixel dimensions after I drew the area I'm interested in. Can it be done?