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The Feed Dialog doesn’t allow including of a pre-set message any more, so you’d have to make that post via the Graph API if you want to pre-fill the message (and even then, you should only do so, if you’ve given the user the possibility to edit the pre-filled message first).
See here for how to make a post on a user’s behalf via Graph API, https://developers.facebook.com/docs/reference/api/user/#posts
You can do that all client-side, if you embed the JavaScript SDK into your page.
You need to set up an app on FB, then have the user connect to it (using FB.login), ask for the necessary permission (publish_stream) while doing so, and after successful login use FB.api to make the Graph API call.
Source:stackexchange.com