403 Invalid token error on GET user info - desire2learn

Question

UPDATE: I thought I had to pass the parameters as a JSON string in the request body, but actually I need to put them on the URL (the endpoint string), so it's working now.
I'm new to Valence. I have some Salesforce Apex code (written by someone else) that creates a D2L user. The code is working fine.
I want to add an Apex method to retrieve info for an existing D2L user using the userName parameter. I've copied the existing method, changed to a GET, set the query parameter to userName, and kept everything else the same.
When I call my method, I get a 403 Invalid Token error.
Do I need to use different authorization parameters for a GET? For example, do I still need to include a timestamp?
Here's a portion of the Salesforce Apex code:
public static final String USERS = '/d2l/api/lp/1.0/users/';
String TIMESTAMP_PARAM_VALUE = String.valueOf(Datetime.now().getTime()).substring(0,10);
String method = GETMETHOD;
String action = USERS;
String signData = method + '&' + action + '&' + TIMESTAMP_PARAM_VALUE;
String userSignature = sign(signData,USER_KEY);
String appSignature = sign(signData,APP_KEY);
String SIGNED_USER_PARAM_VALUE = userSignature;
String SIGNED_APP_PARAM_VALUE = appSignature;
String endPoint = DOMAIN + action + '?' +
APP_ID_PARAM + '=' + APP_ID + '&' +
USER_ID_PARAM + '=' + USER_ID + '&' +
SIGNED_USER_PARAM + '=' + SIGNED_USER_PARAM_VALUE + '&' +
SIGNED_APP_PARAM + '=' + SIGNED_APP_PARAM_VALUE + '&' +
TIMESTAMP_PARAM + '=' + TIMESTAMP_PARAM_VALUE;
HttpRequest req = new HttpRequest();
req.setMethod(method);
req.setTimeout(30000);
req.setEndpoint(endPoint);
req.setBody('{ "orgDefinedId"' + ':' + '"' + person.Id + '" }');

Answer 1

I thought I had to pass the parameters as a JSON string in the request body, but actually I need to put them on the URL (the endpoint string), so it's working now