How can I parse a configuration parameter in kotlin dsl?

I would like to define a new environment variable in my kotlin dsl like this:

project {
  params {
    param("env.MY_NEW_ENV_VAR", compute_env_var_value())
  }
  [...]
}

I would like the compute_env_var_value() method to base on the value of configuration parameter %teamcity.build.branch%. It should parse that value, get the relevant information out of it, and generate a string that is then returned. Something like this, for example:

fun compute_env_var_value(): String {
    val match = Regex("([A-Z]+)-(\\d+)").find("%teamcity.build.branch%")!!
    val (project_name, issue_number) = match.destructured

    return "$project_name-$issue_number"
}

What I noted is that this function does not evaluate the value of variable %teamcity.build.branch%. Instead, it really parses the string "%teamcity.build.branch%", instead of its actual value. Why is that?

Currently, the only solution I've found to this problem is to do the above parsing in a shell script where the %teamcity.build.branch% value is replaced with its actual value. Is there a way to do it in the kotlin dsl at all?

Thanks for your help