For a simple button that just opens your custom GUI, just adding opengui with your custom GUI's identifier (like msgsumc) is usually enough. So, your Edit Action Script menu for that button would look something like this:
▶ Action Script
│
└─■ Open GUI: msgsumc
However, if you're trying to replace the default title screen right when the game starts using a Ticker element, you might want to add a closegui action first. This ensures the current screen (the vanilla title screen) is closed before your custom GUI opens, making the transition cleaner.
For a Ticker on the title screen wanting to open your custom GUI automatically:
▶ Action Script
│
├─■ Close GUI
│
└─■ Open GUI: msgsumc
This way, the game closes the initial screen, then opens yours.
But again, for a regular button you click, just the opengui action is perfectly fine! 😊