The thing is, the msgbox command is just a loadpointer command with some other things. Here's an example of what msgbox @string1 0x4 would actually look like in a FireRed ROM:
0F 00 XX XX XX 08 09 04
Now, 0F is the command ID for "loadpointer" in XSE. The 00 byte tells the command which script bank to access, i.e. script bank 0.
XX XX XX 08 is just a pointer to the string.
09 is the callstd function. In older versions of XSE, callstd was called boxset and the msgbox command simply had a pointer to the string and you had to include the boxset command, but HackMew compacted all of this into one command despite it being a few. Either way, callstd is a separate command which calls certain "standard" scripts in game and uses them, as a way to save bytes upon compilation.
04 simply determines what kind of textbox it is, so therefore which standard script the ROM calls, which is, in this case, the MSG_KEEPOPEN standard script, or standard script 0x4.