Cura Insert Any GCode At Any Layer. Even add multiple lines if you separate them with a comma. From Cura, click on Help, Show Configuration Folder, then copy this file into the scripts folder, restart Cura and you're all done.

RunBeforeLayer.py

from ..Script import Script
import re


class RunBeforeLayer(Script):

    def getSettingDataString(self) -> str:
        return """
        {
            "name": "Run Before Layer",
            "key": "RunBeforeLayer",
            "metadata": {},
            "version": 2,
            "settings":
            {
                "run_layer":
                {
                    "label": "Layer",
                    "description": "At what layer should the code run?",
                    "type": "int",
                    "default_value": "4"
                },
                "custom_gcode":
                {
                    "label": "Code At Layer",
                    "description": "Any custom g-code to run before the layer, for example, M300 S440 P200 to beep. Separate multiple commands with a comma.",
                    "type": "str",
                    "default_value": "Code"
                }
            }
        }
        """

    def execute(self, data):
        layer = self.getSettingValueByKey("run_layer") + 2
        gcode = re.sub("\\s*,\\s*", "\n", self.getSettingValueByKey("custom_gcode"))
        lines = data[layer].split("\n")
        lines.insert(1, gcode)
        data[layer] = "\n".join(lines)
        return data