The official doco allows you to switch the examples between the Groovy and Kotlin DSLs. Currently the answer listed there to your question is:

repositories {
    mavenCentral()
    maven {
        url = uri("<MAVEN REPO URL>")
    }
}

I needed to add Gitlab with authentication, which has a more complicated syntax. For others that stumble upon this, here is the official Gitlab example translated to the kts/Kotlin syntax.

val gitLabPrivateToken: String by project

maven {
    url = uri("https://<gitlab-url>/api/v4/groups/<group>/-/packages/maven")
    name = "GitLab"
    credentials(HttpHeaderCredentials::class) {
        name = "Private-Token"
        value = gitLabPrivateToken
    }
    authentication {
        create<HttpHeaderAuthentication>("header")
    }
}

The example URL here is true to Gitlab doco. But for me, it only worked with a URL like this:
https://gitlab.com/api/v4/projects/12345/packages/maven