How to use a variable in headers when making a POST request in Red

I'm trying to make a POST request using Red language.

I have to pass a header with authorization, and I'm computing the value for the authorization string before making the request and saving it in a variable auth-string.

probe prints the auth-string value fine.

If I hard-code the authorization string in the headers section, it works, but when I try to use the variable the program exits.

This code with hard-coded Authorization: "Basic c29tZX... works:

username: "someusernamestring"
password: "somepasswordstring"

url: to url! rejoin ["https://example.com/" username "/Account.json"]

auth-string: rejoin ["Basic " enbase/base rejoin [username ":" password] 64]
probe auth-string

response: write url [
        post [
        Content-Type: "application/x-www-form-urlencoded"
        Authorization: "Basic c29tZXVzZXJuYW1lc3RyaW5nOnNvbWVwYXNzd29yZHN0cmluZw=="
    ]
    {}    
]

print response

But, this code, with the variable Authorization: auth-string does not work, the program exits, no error that I can see.

username: "someusernamestring"
password: "somepasswordstring"

url: to url! rejoin ["https://example.com/" username "/Account.json"]

auth-string: rejoin ["Basic " enbase/base rejoin [username ":" password] 64]
probe auth-string

response: write url [
        post [
        Content-Type: "application/x-www-form-urlencoded"
        Authorization: auth-string
    ]
    {}    
]

print response

How do I use a variable in the headers section?

Solution

This should work as it does in Rebol

response: write url compose/deep [
     post [
        Content-Type: "application/x-www-form-urlencoded"
        Authorization: (auth-string)
    ]
    {}    
]