In a GET method the server isn't expecting a "payload" (what you would specify in the IN= option on PROC HTTP), so you would place the credentials argument on the URL or...if it's Basic Auth...on the WEBUSERNAME/WEBUSERPASSWORD options.
filename resp temp;
proc http
url="https://httpbin.org?user=myusername&password=mypassword&token=ABC1234"
method='GET'
out=resp;
run;
Sometimes, the API expects a token to be specified as part of the header, which you place in the HEADERS statement:
/* authtoken retrieved in previous step */
filename resp temp;
proc http
url="https://httpbin.org/api/2.0/search?q=&query."
method="GET"
out=resp
;
headers
"Authorization"="Bearer &authtoken.";
run;
This webinar/article covers a lot of the basics.
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