Creating global variable and referencing it in power query

Hello Folks,

I need your help in creating a global variable which will store a text value (known as access token), which will be used to assign a value to the headers in the Web.Contents function in my power query, As I am doing this because my access token expires in every 1 hour and I need to generate it manually via POSTMAN and have to replace that value in all queries as the data source for all queries are same i.e. web service, So I need to replace the access token value in all my queries every time I regenerate it which will takes a lot of time.

So, in order to replace the values in each query and I want to create a global variable which will store my text value and can work dynamically in all my queries and that needs to be changed at a single place rather than changing it everywhere. 

I want expected result like below:-

let

Source = Json.Document(Web.Contents("URL", [Headers=[Authorization="variable"]]))

in 

source