Create your custom button for this purpose then registor click event in javascript. Inside that event use FB.UI method of Fabebook API
https://developers.facebook.com/docs/sharing/reference/share-dialog[
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$('#shareButton').click(function() {
FB.ui({
method: 'share',
link: 'samplelinktoshare',
caption: 'An example caption',
}, function(response){
if (response === null) {
console.log('was not shared');
} else {
}
});
});