1. Click the "Add Vector Layer" button
2. Change "Source type" from "File" to "Protocol"
3. Ensure "Type" is "GeoJSON"
4. Paste in your URL
5. Click "Open" (or "Add" in QGIS 2.99dev/3.0)

The below pics are from QGIS 2.99dev (what will be QGIS 3.0) but 2.18 is basically the same.

1. Click the "Add Vector Layer" button
2. Change "Source type" from "File" to "Protocol"
3. Ensure "Type" is "GeoJSON"
4. Paste in your URL
5. Click "Open" (or "Add" in QGIS 2.99dev/3.0)

The below pics are from QGIS 2.99dev (what will be QGIS 3.0) but 2.18 is basically the same.

1. Click the "Add Vector Layer" button
2. Change "Source type" from "File" to "Protocol"
3. Ensure "Type" is "GeoJSON"
4. Paste in your URL
5. Click "Open" (or "Add in QGIS 2.99dev/3.0)

The below pics are from QGIS 2.99dev (what will be QGIS 3.0) but 2.18 is basically the same.

1. Click the "Add Vector Layer" button
2. Change "Source type" from "File" to "Protocol"
3. Ensure "Type" is "GeoJSON"
4. Paste in your URL
5. Click "Open" (or "Add" in QGIS 2.99dev/3.0)

The below pics are from QGIS 2.99dev (what will be QGIS 3.0) but 2.18 is basically the same.