How to correctly describe the snap section for electron-builder.json?

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Can you please tell me how to correctly specify the settings for the snap section in electron-builder.json ? If the application uses third-party applications. In mine it's ffmpeg and graphicsmagick (gm).
For example, for compilation in deb there is a depends block and it works fine. The final deb contains these dependencies, everything works fine. However, in the case of snap , there are no such dependencies (or access to them).
My electron-builder.json:

{
  "productName": "sht-vr-player",
  "appId": "cf.sht-vr-player",
  "directories": {
    "buildResources": "build_resources"
  },
  "linux": {
    "target": [
      "deb",
      "snap"
    ],
    "icon": "icon/current",
    "synopsis": "..",
    "category": "Graphics",
    "description": "..long text.."
  },
  "deb": {
    "depends": [
      "ffmpeg",
      "graphicsmagick"
    ]
  },
  "snap": {
    "confinement": "strict",
    "stagePackages": [
      "default",
      "ffmpeg",
      "graphicsmagick"
    ],
    "plugs": ["default", "ffmpeg","gm"]
  }

}

Is it related to the strict parameter and access is only possible with the value classic ?
ps: By the way, when adding the stagePackages parameter, compilation does not occur in principle (well, or I could not wait for it, even after several hours). However, snapcraft.yaml generates the required files and the manual compilation snapcraftruns successfully. Have you faced such a problem?
Thank you!