Signature Method : HMAC-SHA256

Java has a hmac support: https://www.baeldung.com/java-hmac

You could use Groovy script test step to calculate hmac from request payload.

I have not personally used HMAC but I could find this site that describe HMAC process and how it works in general level: https://dotnettutorials.net/lesson/hmac-authentication-web-api/

You just have to follow a client process with SoapUI requests and then do required calculations by utilizing groovy scripts.