How do I add a built-in Resourcepack to my Mod?

Add it to the correct folder in runs folder in your workspace folder.

Runs folder is like regular MC install folder

Do you have permission to use those files?

Resources as all other intellectual property are copyrighted

What he's trying to do and same as I am, is like myself I offer a dark mod option what you go in to the resource folder and enable it just like minecraft does for programers art many mods seem to do it in forge but I not getting any luck to get it to work in neoforge I think something changes in 1.20.x where we have to register the resource in the mod but nothing I try works..

So like him I am also trying to registed a second texture pack what isn't used untl they enable it what then would overirde my mods textures

I couldn't get it to work or it changes a lot between versions so I just copy the zip

Warning this is a hacky way of doing it, just everything I tried it didn't work so I have in to do it away I know it should work.

Create a custom code element and then grab the name from the public class then chance CUSTOMCODENAME to your new name, then change net.namespace.modid; to your namespace/modid, you will find what you need at top of your MCreator custom code page
removed all the old stuff with this below with the above changes.

1.21.1/4 @EventBusSubscriber(bus = EventBusSubscriber.Bus.MOD)
1.21.8 @EventBusSubscriber

package net.namespace.modid; //change me
import org.slf4j.Logger;
import net.neoforged.fml.event.lifecycle.FMLCommonSetupEvent;
import net.neoforged.fml.common.EventBusSubscriber;
import net.neoforged.bus.api.SubscribeEvent;
import java.nio.file.Paths;
import java.nio.file.Path;
import java.nio.file.Files;
import java.io.InputStream;
import com.mojang.logging.LogUtils;

@EventBusSubscriber(bus = EventBusSubscriber.Bus.MOD)
public class CUSTOMCODENAME { //Change this
    private static final Logger LOGGER = LogUtils.getLogger();

    @SubscribeEvent
    public static void onSetup(FMLCommonSetupEvent event) {
        Path resourcePacksDir = Paths.get("resourcepacks");
        Path targetPack = resourcePacksDir.resolve("pulpe_faction_darkmode.zip");
        if (!Files.exists(targetPack)) {
            try (InputStream is = CUSTOMCODENAME.class.getResourceAsStream("/pulpe_faction_darkmode.zip")) { //Change this
                if (is != null) {
                    Files.createDirectories(resourcePacksDir);
                    Files.copy(is, targetPack);
                    LOGGER.info("Copying Custom Resource Pack..."); //Change this if you want
                }
            } catch (Exception e) {
                e.printStackTrace();
                LOGGER.error("Error Copying Resource Pack...", e); //Change this if you want
            }
        }
    }
}