How to use a fetched access token to fetch an endpoint?

How can I use (insert) a (just) fetched access token in Javascript to get access to an endpoint (access ID is to be added to the endpoint URI – oauth2 – Strava API).

I tried to add the fetched access token as const and var to the URI of the endpoint, but I do not get any data returned. All is working if I manually insert the actual access_code in the URI (FETCHED_ACCESS_TOKEN), but how do I pick it up to be used automatically in the endpoint URI?

JAVASCRIPT CODE I USED:

let _data = {
    client_id:"11111",
    client_secret:"AAAAAAAAAAAAAAAAAAAAAAAAAAAA",
    code:"exchange_code",
    grant_type:"authorization_code"
}

fetch("https://www.strava.com/api/v3/oauth/token", {
  method: "POST",
  body: JSON.stringify(_data),
  headers: {"Content-type": "application/json; charset=UTF-8"}
})
.then(response => response.json()) 
.then(json => console.log(json))
.then(json => {data = json})
.then(res => getActivities(res))
.catch(err => console.log(err));


const auth_link = "https://www.strava.com/oauth/token"

async function getActivities(res) {

    const activities_link = "https://www.strava.com/api/v3/segments/11111111?access_token=FETCHED_ACCESS_TOKEN"

                await fetch(activities_link)
                    .then(res => res.json())
                    .then(json => console.log(json));

            }