build.gradle

buildscript {
    repositories {
        // These repositories are only for Gradle plugins, put any other repositories in the repository block further below
        maven { url = 'https://maven.minecraftforge.net' }
        mavenCentral()
    }
    dependencies {
        classpath group: 'net.minecraftforge.gradle', name: 'ForgeGradle', version: '5.1.+'
    }
}

apply plugin: "net.minecraftforge.gradle"

minecraft {
    mappings channel: 'official', version: '1.18.1'
}

dependencies {
    minecraft 'net.minecraftforge:forge:1.18.1-39.0.14'
}

build.gradle.kts

buildscript {
    repositories {
        // These repositories are only for Gradle plugins, put any other repositories in the repository block further below
        maven(url = "https://maven.minecraftforge.net")
        mavenCentral()
    }
    dependencies {
        classpath(group = "net.minecraftforge.gradle", name = "ForgeGradle", version = "5.1.+")
    }
}

apply(plugin = "net.minecraftforge.gradle")

minecraft { // <- this gives "Unresolved reference: minecraft". Is there a way to use quotes (untyped I think its called, when I try it thinks im trying to invoke string)
    mappings(channel = "official", version = "1.18.1") // <- this gives "Unresolved reference: mappings"
}

dependencies {
    "minecraft"("net.minecraftforge:forge:1.18.1-39.0.14") // This seems to work by using quotes.
}