When you press this button, you will see this window:
You can choose to activate the hook by the template (not controllable by the user) or by a user variable (controllable at run-time). When you choose "Changed by: Template", then the "Template Hook" button is enabled, otherwise the "Hook by Variable" button is.
When you click on the "Template Hook" button, you will see this window:
Here you activate the hook to the message box replacement. When you activate this option, the "With procedure:" option on the previous window will activate as well. Then, you have to fill that option.
The drop list presents a list with all your procedures. Pick the one referred at Creating a Message Replacement.
If you pick the "User variable" option instead, you may click on the "Hook by Variable" button.
Then, you will see the following window, where you have to select a global variable to hold the value of the activation status.
The variable will hold a True or False value, to activate or not the replacement (A BYTE type variable will do it).
To shorten the variable drop list, it will shown only the global variables that contains "hook" as part of the name, case insensitive (for example, "GLO:MessageHook").
You have to load this variable at run-time (you may use the --- code template) and activate the hook in an embed with the --- code template for the replacement to work.