Using curl command for react native to fetch api

I'm trying to use the petfinder APi for an app I'm creating and following the API documentation which can be found here : https://www.petfinder.com/developers/v2/docs/#developer-resources.

It gives the command : curl -d "grant_type=client_credentials&client_id={CLIENT-ID}&client_secret={CLIENT-SECRET}" https://api.petfinder.com/v2/oauth2/token

Im trying to translate this for react native and used the following code:

getAdopt1 = async() => {
fetch('https://api.petfinder.com/v2/oauth2/token', {
method: 'POST',
headers: {
Accept: 'application/json',
'Content-Type': 'application/json',
},
body: JSON.stringify({
firstParam: "grant_type=client_credentials&client_id={CLIENT-ID}&client_secret={CLIENT-SECRET}"
}),
}).then((response) => response.json())
.then((responseJson) => {
let res = JSON.stringify(responseJson)
console.log("Response: "+res)
return responseJson;
})
.catch((error) => {
console.error(error);
})
}

However I get the following error:

Response: {"type":"https://httpstatus.es/400","status":400,"title":"unsupported_grant_type","detail":"The authorization grant type is not supported by the authorization server.","errors":[{"code":"unsupported_grant_type","title":"Unauthorized","message":"The authorization grant type is not supported by the authorization server. - Check that all required parameters have been provided","details":"The authorization grant type is not supported by the authorization server. - Check that all required parameters have been provided","href":"http://developer.petfinder.com/v2/errors.html#unsupported_grant_type"}],"hint":"Check that all required parameters have been provided"}

What am I doing wrong here ?