Facebook does not allow any app to pre-populate a post with "content that the user didn’t enter, even if the text can be edited or deleted before sharing." When sharing to Facebook, users will need to create their own share text and disclose their connection to your program manually. You can learn more about Facebook's policy here.
Add the attached copy and image to a content card to let your users know that they'll need to write their own message.
Keep an eye on other social networks that your users share to frequently, as they may also restrict the use of pre-populated messages. In that case, adapt the example content as needed.