How to set markdown snippet trigger automatically

There is an inconsistency in VS Code and it is a known issue. Markdown snippets do not get auto-completion. Probably it will be fixed in next release.

Yes this is an inconsistency. Full extensions get auto (7x24) completion by default (e.g. latex, cake), some built-in extensions like Markdown do not.

https://github.com/Microsoft/vscode/issues/1617#issuecomment-166999086

Try setting:

"[markdown]": {
  "editor.quickSuggestions": {
    "other": true,
    "comments": false,
    "strings": true
  },
},

This enables IntelliSense automatically when you start typing. By default, this will show snippets and word based suggestions (suggestions based on words in the current document). To disable word based suggestions, set:

"[markdown]": {
    "editor.quickSuggestions": true,
    "editor.wordBasedSuggestions": false
}

You can also set:

"editor.snippetSuggestions": "top"

If you always want snippets to show before word base suggestions