How to call a SAS STP in background using proc http and http_tokenauth

I'm trying to call a stored process (STP) from SAS via proc http using the option background, to ensure that my main process does not wait for the STP to be finished. I do use the following code:

filename resp "<path to response file>";
%let url = https://<ServerName>:<port>/SASStoredProcess/do;
%let question_mark = ?;

proc http
url="%str(&url.)&question_mark.%nrstr(_action=background&_program=<path to STP>&num1=11&num2=22)"
out=resp
http_tokenauth;
HEADERS "Content-Type"="application/x-www-form-urlencoded" "Accept"="application/json";
quit;

Unfortunately, in my response file I do always receive the SAS logon page.

Sending username/pw as part of my URL does work, but it is not wanted for security reasons. Hence I need to use http_tokenauth, which works for non background STPs. Also, when I just paste my URL to the browser, it does work.

How can I ensure it does also work using proc http and http_tokenauth?

Additional Information:

Setup:

I do have a STP which is called by a process engine via REST and expects an immediate answer containing an ID.(Main STP) This main STP then needs to call a sub STP containing a process, which will approx. take one hour to be finished. Hence I do not want the Main STP to wait for the sub STP to be finished. Therefore the sub STP should be called in background.

Solution

Token authentication only works when using "do1" instead of "do". Referring to documentation: https://documentation.sas.com/?docsetId=stpug&docsetTarget=n0ax7wadghvldwn1bbarfuly6adl.htm&docsetVersion=9.4&locale=en

Furthermore I'm using query to pass my parameters, which makes it more clear.

So this does work now:

%let url = https://<ServerName>:<port>/SASStoredProcess/do1;

proc http 
  url="%str(&url.)"
  query=("_action"             = "background"
         "_program"            = "<path to STP>"
         "num1"                = "1"
         "num2"                = "2"
         )
  ct="application/x-www-form-urlencoded"
  http_tokenauth
  out=resp;
quit;