Under "Manage Jenkins / Configure System",
I configured "Mask Passwords - Parameters to automatically mask" for:
Active Choices Reactive Reference Parameter
Active Choices Reactive Parameter
Active Choices Parameter

and created one "Mask Passwords - Global name/password pairs" for:
build_password

In Jenkins job configuration,
I have an "Active Choices Reactive Parameter" using Groovy script
and tries to use that global variable ${build_password}:

if ( SVN_FOLDER.endsWith("/trunk") ){
    return ["N/A"]
} else {
    def SVN_LIST_URL = "${SVN_ROOT}/${SVN_FOLDER}"
    def SVN_CMD_ARG = "svn ls --username build --password ${build_password} --non-interactive ${SVN_LIST_URL}"
    def SVN_CMD_OUT = SVN_CMD_ARG.execute().text
    def SVN_SELECCTION_LIST = SVN_CMD_OUT.split('/\n').toList().sort().reverse()

    return SVN_SELECCTION_LIST
}

The above code works only if I replace ${build_password} with actual password string.