var projName = "My First Auto-created Project";

var app = Application("Omnifocus")
app.includeStandardAdditions = true
var doc = app.defaultDocument
var matchingProjects = doc.flattenedProjects.whose({name: projName})
var theProject = null
if (matchingProjects.length == 0) {
  theProject = app.Project({
    name: projName, 
    status: 'active'
  })  
  doc.projects.push(theProject)
  app.displayDialog("Created new project")
} else {
  theProject = matchingProjects[0]
  app.displayDialog("Reused existing project")
}
//theProject is now set (whether created new or reading from match)