This or this? Not sure if either can be toggled by a button. Also is the map for TF2? Buttons don't work in TF2 unless set to activate via another method, ie. when shot.

As for the Portal model, that may be a texture or effect, I don't think it's a model...but I could be wrong.

Create a trigger, name the trigger, lets say "triggername".

Next create the button and add the following output:

On Pressed; triggername; enable;

After this create an env_hudhint, name it (ex. hudhintname), and insert the desired text you want to show to these players inside the trigger.

Next, go on the trigger and add the following output:

On Start Touch; hudhintname; Show;

Whenever someone enters the trigger while its enabled, a message will show in their hud.

PS: Just noticed this is for TF2, never tried it in this game and I can't think of another way to do it.