UPS OAuth2.0 Access token request with SQL in RPG-Program "Invalid/Missing Authorization Header" with BASE64ENCODE

I have the same problem like Brian here: UPS API OAuth token request fails.The try it out in Postman works. I get my Access Token.My task now is to make a working SQL statement also for UPS API.

 SELECT CAST(RESPONSEMSG AS CLOB(1000000)),             CAST(responseHttpHeader  AS CLOB(1000000))       INTO :W_RSP, :W_RSPHDR        FROM TABLE( SYSTOOLS.HTTPPOSTCLOBVerbose(                  CAST(:W_URL AS VARCHAR(255)),                  CAST (:HEADER AS CLOB(1K)),                  CAST (:PAYLOAD AS CLOB(20K)))                 ) AS LOGIN; W_URL = 'https://wwwcie.ups.com/security/v1/oauth/token';HEADER ='<httpHeader><header name="Authorization" value="Basic '+ W_BS64 +'"/><header name="accept" value="application/json"/><header name="Content-Type" value="application/x-www-form-urlencoded"/><header name="x-merchant-id" value="xp1g0NaY2hAqDZbkoNAeDs8X3R1bH72KCdmNUeV5DYOMlfiU"/></httpHeader>'; PAYLOAD = 'grant_type=client_credentials';

And I encode the W_BS64 like this:

W_Client = '{xxxxclientIDxxxx}:{xxxxxclientsecretxxxxx}'Values QSYS2.BASE64_ENCODE(:W_Client) Into :W_BS64;  

When I debug this in my SQLRPG program, W_RSP is empty and W_RSPHDR includes only weird characters.When I use the ACS SQL Scripter and use the BASE64 String in the header, I also get the "Invalid/Missing Authorization Header" Error. So I don't know if it is because of the BASE64 is not right, or if I am missing something.I thought I did the conversion from the UPS Documentation curl to SQL correctly.

I have not found an example how to use SQL to get a successful response with the Access token yet.Please let me know, what I am doing wrong.