How can I add a custom status message to be shown on the Event page replacing the register button?

On the event overview page, there are all the possible options to be set for an event such as Published, Draft, test, Make it Live and so on. Among these, there is an option named  "Custom". Once this is selected, a popup comes out that allows you to enter the text message that has to be displayed. Once this is done,  on the event landing page,  but will become inactive and the added message will show up.