Convert REST curl command to Apex Request - rest

Question

I have a curl example of a REST API call as follows
curl https://api.endpoint.com/api_action.json \
-u key:secret \
-d 'message=Hello World' \
-d id=12345
Now I'm trying to replicate this using the Apex HttpRequest class but I'm unsure as to how I should be passing the options into the call.
What I have so far is as follows
HttpRequest req = new HttpRequest();
req.setEndpoint(https://api.endpoint.com/api_action.json);
req.setMethod('POST');
// what goes in these
req.setHeader(stuff);
req.setBody(stuff);
Http http = new Http();
HttpResponse res = http.send(req);

Answer 1

From #sfdcfox answer
Given this comment:
curl -i -X POST --data "first_name=test&last_name=testlast&email=test#test.com&phone=1234567&company=Ac‌​meInc&years_in_business=3&amount=4000&dealer_id=524b2833f7494317db000001" https://example.com/webform/start/
The code you need would be approximately:
String payload='first_name=test&last_name=testlast&email=test#test.com&phone=1234567&company=Ac‌​meInc&years_in_business=3&amount=4000&dealer_id=524b2833f7494317db000001';
HttpRequest req = new HttpRequest();
req.setMethod('POST');
req.setEndpoint('https://example.com/webform/start/');
req.setHeader('Content-Type','application/x-www-form-urlencoded');
req.setHeader('Content-Length',String.valueOf(payload.length()));
req.setBody(payload);
Http binding = new Http();
HttpResponse res = binding.send(req);

Answer 2

When you are using APEX, you can use the build-in apex_web_service.make_rest_request to do this:
declare
v_clob clob;
begin
v_clob := apex_web_service.make_rest_request(
p_url => <your URL>,
p_http_method => 'POST',
p_body => <whatever body>,
p_username => 'username',
p_password => 'password' );
htp.p(v_clob);
end;