By Javascript you can pop-up the media and on select you can update the image URL to the textbox. Below is the code.
var custom_uploader;
jQuery('<Should-be-a-button-selector>').click(function(e) {
e.preventDefault();
//If the uploader object has already been created, reopen the dialog
if (custom_uploader) {
custom_uploader.open();
return;
}
//Extend the wp.media object
custom_uploader = wp.media.frames.file_frame = wp.media({
title: 'Choose Image',
button: {
text: 'Choose Image'
},
multiple: false
});
//When a file is selected, grab the URL and set it as the text field's value
custom_uploader.on('select', function() {
attachment = custom_uploader.state().get('selection').first().toJSON();
jQuery('#Cat_meta[img]').val(attachment.url);
//Open the uploader dialog
custom_uploader.open();
});