I'm assuming that you're using the ConnectApiHelper class. It's not officially supported by Salesforce, although it was initially written by me (I'm a Salesforce employee).
Currently there isn't an option to control the visibility, but since the source code is available, it's easy to add. Look for the code that creates the
ConnectApi.FeedItemInput input = new ConnectApi.FeedItemInput();
input.body = messageInput;
input.subjectId = subjectId;
You'll want to add a parameter that lets you specify the
ConnectApi.FeedItemVisibilityType. It can be either
ConnectApi.FeedItemVisibilityType.InternalUsers (the default) or
input.visibility = your_new_parameter
Here are the Apex docs about ConnectApi.FeedItemVisibilityType and ConnectApi.FeedItemInput.
I added this as an issue in the ConnectApiHelper GitHub repo. I'll address it when I have some free time, unless someone beats me to it. (Feel free to send a pull request my way!)