{
  "connections": {}
}

jq --arg ssid "$1" --arg pass "$2" '.connections.$ssid = "$pass"' file.json > tmp.json && mv tmp.json file.json

{
  "connections": {key:"value"}
}

jq --arg ssid abc --arg pass 123 '.connections[$ssid] = $pass' < file.json
{
  "connections": {
    "abc": "123"
  }
}