How do I specify a custom display resolution in Windows 10?

First of all, this question is about inputting a display resolution that doesn't exist on any physical monitor like e.g. 1234x5678, not about picking from the dropdown list.

Secondly, no help from the (barebones) graphics driver can be expected here. Everything must be done on the OS level.

Finally, this is a VM so any resolution can be supported as it's basically just painting a window. Heck, the current resolution is 1046x1527 and my goal is to shorten the window to maybe 1046x1200ish without spending hours dragging window borders 1 pixel at a time trying to get it just right.