I
think this may be exclusive to DMaps.
On the DMap "Appearance" tab, there's a "DMap Intro" text box, which is basically a single string entry exclusive to that DMap. It will display on whatever screen you first access after entering the DMap. This is how people do dungeon intros, if that's what you're attempting. By default, this message only displays once. However, on the DMap "Flags" tab, near the bottom, there's a checkbox for "Always Display Intro String" if you want it to display every time you enter the DMap.
Obviously this is rather limited in that you can only set the screen that displays the message by making it an entrance warp destination. You
could get into sidewarp shenanigans by splitting the level into multiple DMaps and connecting those DMaps with scrolling warps if you want these strings to appear in various other screens throughout your level, but at that point you're best off using a script like
Message Script by Hoff123. This will let you display a string on any screen, and provides a number of options like whether the string shows on first entry or always, whether you want the same string every time or an alternate string the second time, whether you want them to display immediately or have a delay, whether a string only shows if you have a certain item, etc.