Using the given variable parameters and putting them in a JSON format body in M

Dear PowerBI wizzards,

Im pretty new to M and its coding. Currently im working to make a presentation with its input comes from an API. The API itselfs needs to be provided with a ClientIdentifier and ClientSecret. When this is done it, it returns me an API acces key wich i then can use to get the data that i want.

The problem that im running into is that i need to get data from multiple clouddirectorys. These different directorys all have differnt credentials.

This is my M code to call the API to give me the API Acces Key that i need:

//(UserName as text, OfficeCode as text, Password as text)=>

let
    url = "https://xxxxxxxxxxxxxxx",
   body = 
    "{""clientIdentifier"":""XXXXXXXX"",
        ""clientSecret"":""XXXXXXXX"",
        ""username"": ""XXXXX"",
        ""password"": ""XXXX"",
        ""officeCode"": ""XXXXXX"",
        
        
        }",
 
    Source = Json.Document(Web.Contents(url,[

           Headers = [#"Content-Type"="application/json"],

           Content = Text.ToBinary(body)] ))
in
    Source

In the JSON body i hardcoded the credentials need to provide me the acces key. Now i want to re-use the code above to give me an acces key for every directory but im failing to combine the variables into the JSON body. Im looking for someting like this:

body = "{""username"": UserName}".

So my question is how can i implement the given paratamers in the top of the code commented out into my JSON body?

Much thanks in advance already!!!