Not sure if the below settings will help, give a try
The default HTTP password that Integration Server must use when invoking a service as
Specifies the default authenticated HTTP user name that Integration Server will use
while acting as a client to invoke a service. For example, if Integration Server is invoking
the pub.client:http service without specifying the auth /user parameter, then Integration
Server uses the value of this property as the user name. There is no default value.
How about passing the user and password with the help of substring service to Auth parameter in pub.client:http?