Assign a workspace to a specific Output (Display)

In order to configure i3 to assign a workspace to a given monitor, first find the device name for the monitor you wish to use with xrandr:

$ xrandr --listmonitors
Monitors: 1
 0: +*eDP-1 1920/309x1080/173+0+0  eDP-1

In this case, the output name is eDP-1. Next add a line like this into a staged copy of your i3 config file:

workspace "$ws1" output DP1

Log back in for the changes to take effect.