Keycloak Missing form parameter: grant_type

You should send your data in a POST request with Content-Type header value set to application/x-www-form-urlencoded, not json.

With Curl

curl -X POST \
http://localhost:8080/auth/realms/api-gateway/protocol/openid-connect/token \
-H 'Accept: */*' \
-H 'Accept-Encoding: gzip, deflate' \
-H 'Cache-Control: no-cache' \
-H 'Connection: keep-alive' \
-H 'Content-Length: 73' \
-H 'Content-Type: application/x-www-form-urlencoded' \
-H 'Cookie: JSESSIONID=F8CD240FF046572F864DC37148E51128.a139df207ece;   JSESSIONID=65D31B82F8C5FCAA5B1577DA03B4647C' \
-H 'Host: localhost:8080' \
-H 'Postman-Token: debc4f90-f555-4769-b392-c1130726a027,d5087d9f-9253-48bd-bb71-fda1d4558e4d' \
-H 'User-Agent: PostmanRuntime/7.15.2' \
-H 'cache-control: no-cache' \
-d 'grant_type=password&client_id=api-gateway&username=admin&password=temp123'

By Postman (Select x-www-form-urlencoded option for parameters)

For those who landed here from a search looking for JavaScript solution.

Here is an example when exchanging code for access_token with keycloak authority using axios.

Sending the request:

const params = new URLSearchParams({

    grant_type: 'authorization_code',
    client_id: 'client-id-here',
    code: 'code-from-previous-redirect',
    redirect_uri: location.protocol + '//' + location.host

});

axios({

    method: 'post',
    url: 'https://my-keycloak.authority/token',
    data: params.toString(),
    config: {
        headers: {'Content-Type': 'application/x-www-form-urlencoded'}
    }

}).then(response => {

    console.log(response.data);

}).catch(error => {

    console.error(error);

});

You are required to send a POST request with the parameters as a URL encoded string in the request body.

FormData object does not work.

For those having problems with curl the curl command is as follows

curl -d "client_secret=<client-secret>" -d "client_id=<client-id>" -d "username=<username>" -d "password=<password>" -d "grant_type=password" "http://localhost:8080/auth/realms/<realm-name>/protocol/openid-connect/token"

The curl command works without the Content-Type header.