There are two web services protocols, to consume with Genexus: SOAP and REST
In order to consume, there are two simple steps
1- Inspecting the wsdl: when thinking of invoking a web service you have to read, at first, its wsdl using the WSDL Inspector (given by an Url or physical path file)
2- Consuming :Once the service has been inspected you can start programming the consumer (using the External Object created automatically)
There is a simple example from Consuming SOAP web services in GeneXus
In order to consume webservice under HTTPS please refer here
In case of needed to set up SOAP Headers, please refer Setting up SOAP Headers
In order to Consume WCF Services ( Ws*), please refer WCF with GeneXus
For more information please refer SOAP Webservices
In order to consume, there is a simple example from Consuming REST Data Providers in Genexus
For more information please refer REST Web Services