This feature is coming Summer '19 (release notes)

In your scratch org definition file (e.g. project-scratch-def.json)

{
  "orgName": "Example",
  "edition": "Enterprise",
  "release": "Preview"             // Preview, Current or Previous
}    

This feature was added in Summer '19 (release notes)

In your scratch org definition file (e.g. project-scratch-def.json)

{
  "orgName": "Example",
  "edition": "Enterprise",
  "release": "Preview"             // Preview, Current or Previous
}