Tagging with AJAX in select2

Select2 has the function “createSearchChoice”:

Creates a new selectable choice from user’s search term. Allows
creation of choices not available via the query function. Useful when
the user can create choices on the fly, eg for the ‘tagging’ usecase.

You could achieve what you want by using:

createSearchChoice:function(term, data) {
  if ($(data).filter(function() {
    return this.text.localeCompare(term)===0;
  }).length===0) {
    return {id:term, text:term};
  }
},
multiple: true

Here’s a more complete answer that returns a JSON result to an ajax search, and allows tags to be created from the term, if the term returned no results:

$(".select2").select2({
  tags: true,
  tokenSeparators: [",", " "],
  createSearchChoice: function(term, data) {
    if ($(data).filter(function() {
      return this.text.localeCompare(term) === 0;
    }).length === 0) {
      return {
        id: term,
        text: term
      };
    }
  },
  multiple: true,
  ajax: {
    url: '/path/to/results.json',
    dataType: "json",
    data: function(term, page) {
      return {
        q: term
      };
    },
    results: function(data, page) {
      return {
        results: data
      };
    }
  }
});