how to add a variable in postman code example

Example 1: how to define variables in api

-Use pm.globals to define a global variable:

pm.globals.set("variable_key", "variable_value");

-Use pm.collectionVariables to
define a collection variable:

pm.collectionVariables.set("variable_key", "variable_value");

-Use pm.environment to define an environment
variable (in the currently selected environment):

pm.environment.set("variable_key", "variable_value");

-You can use unset to remove a variable:
pm.environment.unset("variable_key");

-Defining local variables
Local variables are temporary values you set
in your request scripts using the following syntax:

pm.variables.set("variable_key", "variable_value");

Example 2: collection variables postman

Only accessible within the same collection and
can not be accessed anywhere else
COLLECTION variables are available throughout 
the requests in a collection and are independent
of environments, so do not change based on the selected environment.
I use collection variables when
I use single environment like url details.
We can refer to the variable by using {{}}(double curly brace)
and  it can be used anywhere