bdbodger explains how it is done

And you have a entity with the key: script_bodger and a value: ok_this_works

If you tried to run that it would not work why? The keys have to be declared .
You may have noticed that in pak4.pk3 there is a radiant directory and in
that directory there is a file called keys.txt . This is the file that declares
the keys used in the game . For example

// syntax:
// float floatVar
// int intVar
// string stringVar

float script_wait  // Used on patrol nodes to make patrollers wait the specified amount before continuing>>>>>>> 1.58
float script_additive_delay // Additive delay is the # of seconds extra a flood_spawner will wait for each additional spawning. Defaults to 1 second.
float script_delay  // Used as a substitute for delay_min/max.
float script_delay_min
....
int script_health
int script_health_easy
int script_ignoreme  // If this setting exists on a spawner this guy will have his .ignoreme set to true
....
string script_tree  // Additional variable for Stalingrad hill charge
string target
string targetname
string groupname

As you see there are 3 types of variables you can use for keys .float which is a decimal number, int which is or course a whole
number and string . So to use your own variables you need to make a txt file and place it in the radiant directory in your pak .
Don't name it keys.txt or you will overwrite the game keys .For this example I would make a txt file and name it mymap.txt and
this file will have this line in it

string script_bodger // this is my key

That is it just make sure the txt is in the radiant directory of your pak and your script will work

For example if you make a txt file and put it in the radiant directory and put this

string script_bodger

then yes you can use

my_thing = getent ( " this does not work","script_bodger");

and it will look for and find an entity with the key/script_bodger with a value of "this does not work" or in other words my_thing.script_bodger will be " this does not work" or

if(my_thing.script_bodger == "this does not work") will be true if you have made the txt file and put it in the radiant directory