How make select2 placeholder for search input

You can use the event:

select2:opening: Triggered before the dropdown is opened. This event can be prevented

It's enough to add the placeholder attribute in this event:

$(this).data('select2').$dropdown.find(':input.select2-search__field').attr('placeholder', 'My Placeholder')

$('select').select2({
    placeholder: 'Select an option'
}).on('select2:opening', function(e) {
    $(this).data('select2').$dropdown.find(':input.select2-search__field').attr('placeholder', 'My Placeholder')
})

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<link href="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.3/css/select2.min.css" rel="stylesheet" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.3/js/select2.min.js"></script>


<select style="width: 100%;">
    <option value="volvo">Volvo</option>
    <option value="saab">Saab</option>
    <option value="mercedes">Mercedes</option>
    <option value="audi">Audi</option>
</select>

Use event select2:open.

$('#mySelect').select2().on('select2:open', function(e){
    $('.select2-search__field').attr('placeholder', 'your placeholder');
})

I had the same need and I had ended up writing a small extension for the Select2 plugin.

There is a new option for the plugin, searchInputPlaceholder, in order to set a placeholder for the 'search' input field.

Add the following code right after the plugin's js file:

(function($) {

    var Defaults = $.fn.select2.amd.require('select2/defaults');

    $.extend(Defaults.defaults, {
        searchInputPlaceholder: ''
    });

    var SearchDropdown = $.fn.select2.amd.require('select2/dropdown/search');

    var _renderSearchDropdown = SearchDropdown.prototype.render;

    SearchDropdown.prototype.render = function(decorated) {

        // invoke parent method
        var $rendered = _renderSearchDropdown.apply(this, Array.prototype.slice.apply(arguments));

        this.$search.attr('placeholder', this.options.get('searchInputPlaceholder'));

        return $rendered;
    };

})(window.jQuery);

Usage:

Initialize the select2 plugin with the searchInputPlaceholder option:

$("select").select2({
    // options 
    searchInputPlaceholder: 'My custom placeholder...'
});

Demo:

Demo available on JsFiddle.

UPDATE May 9, 2020

Tested with the latest Select2 Version (v4.0.13) - JsFiddle.

Github repo:

https://github.com/andreivictor/select2-searchInputPlaceholder