I need to implement a bi-directional communication between Microsoft Dynamics CRM and a 3rd party server. The ideal scenario is as follows:

1. User tries to create an entity in CRM
2. In pre-create hook a 3rd party library function is called (or web service or whatever), filled with relevant info, which tries to create the respective entity on the server
3. If the call fails, creation fails in CRM
4. If the call succeeds, the entity is created in the CRM AND additional fields are filled with return values from the call

More specifically, I want to do something like this when user tries to create a new entity instance:

try {
    ExternalWebService.CreateTrade(ref TradeInfo info)
    //this was initialized on the external server
    myCRM_Trade_Entity.SerialNo = info.SerialNo; 
    CreateNew(myCRM_Trade_Entity);

} catch (whatever) {
    fail;
}

What would be the suggested way to do this? I am new to Dynamics, have read about Workflows and Plugins but am not sure how should I do this properly.