Security Token to be generated before submission

Hi All,

I am currently working on a system stress test.
However, I do not know what approach to take using soapUI.
Before calling the web service from the Test Suite, I would like to call a customized JAVA class that invoke the security service sending user and password and receives a user security token.
This security token is needed to pass the service's security checks.

For this to work I would need to set this security token in the security header for each call. That is to say, I need that this field is a dynamic field in the XML. Highlighted in red in attached image.

I am at lost as to how to set a step that calls a JAVA class and how the data return by this call (the token) can be set to this dynamic field in the XML.

Hope you can give me some advice
Thanks in advance