The short answer is that you're not supposed to post something on a user's behalf without giving them the option of editing the comment/status. You can do it with an API call (rather than a share sheet), but this should only be used to post content that the user entered elsewhere in the app.
// NOTE: pre-filling fields associated with Facebook posts,
// unless the user manually generated the content earlier in the workflow of your app,
// can be against the Platform policies: https://developers.facebook.com/policy
[FBRequestConnection startForPostStatusUpdate:@"User-generated status update."
completionHandler:^(FBRequestConnection *connection, id result, NSError *error) {
if (!error) {
// Status update posted successfully to Facebook
NSLog(@"result: %@", result);
} else {
// An error occurred, we need to handle the error
// See: https://developers.facebook.com/docs/ios/errors
NSLog(@"%@", error.description);
}
}];
This is from Facebook's "Sharing in iOS" documentation
Another option (though possibly also against the rules) is to create an image from the text and let the user share the image. Before doing this, however, put yourself in the user's shoes - do you really need to post something like that?