How to add a new web_custom_request, how do you know which URL to use?

Hi,

Please help me correlate the value ab.

Inorder to correlate the sessionid I looked at the response under the web_custom_request("login") transaction id 113.  Correlation for jsessionid worked. 
However transaction id 113 does not have the response for ab vallue.  I found the response for ab under transaction id 159.

Now I don't understand where to put my ab correlation.  I belive I'm missing a web_custom_request for transaction id 159,
How can I add a custom request?  how do I know which URL to enter?

//Correlations
  web_reg_save_param_ex(
  "ParamName=jsessionid", 
  "LB=JSESSIONID=", 
  "RB=;",  
  SEARCH_FILTERS,
  "Scope=Cookies",  
  "RequestUrl=*/login*",   
  LAST);

 web_reg_save_param_ex(
  "ParamName=AB",
  "LB=name=\"ab\" value=\"",  
  "RB=\"",
  SEARCH_FILTERS,
  "IgnoreRedirections=Yes",
  "RequestUrl=*/login*", 
  LAST);

//Login request
 web_custom_request("login;jsessionid=saewerwqrqwqrwrwreqwerqwerqwerqwert87o0MfsVw4!196633223",
  "URL=https://mysite/login;jsessionid={jsessionid}?service=https%1A%4F%site%2Fabc%2F22%2Flogon%2Fogon.xhtml%3FtargetWindowName%3DHome",
  "Method=POST",
  "Resource=0",
  "RecContentType=text/html",   
  "Referer=https://mysite/login?service=https%1A%4F%site%2Fabc%2F22%2Flogon%2Fogon.xhtml%3FtargetWindowName%3DHome",
  "Snapshot=t20.inf",
  "Mode=HTTP",
  "Body=username=usertest&password=testtest&ab={AB}&submit=Login",
  LAST);
 

Here's the response for ab under transaction id 159.

<input type="hidden" name="AB" value="LT-1222-44EG555FFFHhtP0t2dPOlvJDSFGKKK3Eeayq" />

Thanks in advance