build.gradle

android {

    (...)
    
    def keystorePropertiesFile = rootProject.file("keystore.properties")
    def keyStoreProperties = new Properties()
    keyStoreProperties.load(new FileInputStream(keystorePropertiesFile))
    
    defaultConfig {
    
        (...)

        buildConfigField("String", "SECRET_TOKEN", keyStoreProperties["SECRET_TOKEN"])
    }
}