Intermission Text BEFORE MAP01

Just create a dummy map like MAP00 in UDMF and write a simple script to end the map instantly on being loaded (OPEN script). Then in MAPINFO, give MAP00 it's own cluster with the exittext you desire. Don't forget the NoIntermission property on MAP00 to skip the stats screen.

If you mean in GZDoom, the way I've done this is to define the intermission script in MAPINFO, then for MAP01, add the Slideshow option (Slideshow = "<intermission name>").  Then as soon as the level starts, use ACS to give the player a SlideshowStarter.

This is how I accomplished it in Freaky Panties.

While not strictly an intermission before MAP01, text and or

scenes at the start of a pwad can be done with a TITLEMAP .

The best example is Knee-Deep in ZDoom ( kdizd_12.pk3 )

for DOOM (not DOOM2). Instead of using text, the story was

conveyed in a voice track.

I have not tested this, but couldn't you give the cluster that starts with MAP01 an enter text?

In Doomsday define a 'before' infine definition like so:

Finale
{
  Before = "MAP01";
  Script {
    Music "victor"
    Flat FLOOR4_8
    Wait 0.5
    Text msg 10 10 "Blah blah"
    WaitText msg
    Wait 7 
    skiphere
    Canskip
    End
  };
}

46 minutes ago, Empyre said:

Doomsday

I like this answer but I'm using GZDoom. I'm thinking the dummy level MAP00 will work though, I never thought of that!

As for entertext I did find this in my research, but there are no examples that I can get to work

I used the MAP00 trick to display an introductory story text. If you'd like an example, look here for the map.

8 minutes ago, dybbuk said:

As for entertext I did find this in my research, but there are no examples that I can get to work

I looked it up here: https://zdoom.org/wiki/MAPINFO/Cluster_definition

Apparently, there has to be a map completed before the cluster begins in order to display the text, so it looks like MAP00 is still going to be required.  Others here have told you how to do it.