Hi Samuel,
You are correct that interfaces are used for sharing data and execution flow between Blueprints. As long as the target pin on the interface message node is connected to a reference to a Blueprint that implements the interface, the function or event in the target will be activated.
In order to get your HUD reference in your “Sender” Blueprint you will need to add a “Get Player Controller” node via the right-click menu. From the “Get Player Controller” node’s Return Value drag off and create a “Get HUD” node, then use the “Get HUD” Return Value as the Target for your BPI. Here is what it would look like:
What I had said about On Tick was incorrect, I apologize. Displaying your Draw Text on your HUD should be done with the Event Receive Draw HUD. Then use Draw Text from that to create your display. The text color by default is Alpha 0 (transparent), so please make sure the input color is visible. You HUD should automatically update with any changes to the string variable.
Please let me know if there is anything else that is not functioning as expected.