If you need to interact directly with the Java keystore (cacerts) that Boomi uses, then the default password for this is “changeit”
It does not seem to be documented anywhere – but it is included in a few of the example keytool commands that can be found in Boomi Community – for instance the one below.
https://community.boomi.com/s/article/Connecting-to-an-IFS-Applications-server-using-a-self-signed-certificate