Edit in JSFiddle

//Original GeoJSON
const maliAsGeoJSON = {"type":"Feature","id":"MLI","properties":{"name":"Mali"},"geometry":{"type":"Polygon","coordinates":[[[-12.17075,14.616834],[-11.834208,14.799097],[-11.666078,15.388208],[-11.349095,15.411256],[-10.650791,15.132746],[-10.086846,15.330486],[-9.700255,15.264107],[-9.550238,15.486497],[-5.537744,15.50169],[-5.315277,16.201854],[-5.488523,16.325102],[-5.971129,20.640833],[-6.453787,24.956591],[-4.923337,24.974574],[-1.550055,22.792666],[1.823228,20.610809],[2.060991,20.142233],[2.683588,19.85623],[3.146661,19.693579],[3.158133,19.057364],[4.267419,19.155265],[4.27021,16.852227],[3.723422,16.184284],[3.638259,15.56812],[2.749993,15.409525],[1.385528,15.323561],[1.015783,14.968182],[0.374892,14.928908],[-0.266257,14.924309],[-0.515854,15.116158],[-1.066363,14.973815],[-2.001035,14.559008],[-2.191825,14.246418],[-2.967694,13.79815],[-3.103707,13.541267],[-3.522803,13.337662],[-4.006391,13.472485],[-4.280405,13.228444],[-4.427166,12.542646],[-5.220942,11.713859],[-5.197843,11.375146],[-5.470565,10.95127],[-5.404342,10.370737],[-5.816926,10.222555],[-6.050452,10.096361],[-6.205223,10.524061],[-6.493965,10.411303],[-6.666461,10.430811],[-6.850507,10.138994],[-7.622759,10.147236],[-7.89959,10.297382],[-8.029944,10.206535],[-8.335377,10.494812],[-8.282357,10.792597],[-8.407311,10.909257],[-8.620321,10.810891],[-8.581305,11.136246],[-8.376305,11.393646],[-8.786099,11.812561],[-8.905265,12.088358],[-9.127474,12.30806],[-9.327616,12.334286],[-9.567912,12.194243],[-9.890993,12.060479],[-10.165214,11.844084],[-10.593224,11.923975],[-10.87083,12.177887],[-11.036556,12.211245],[-11.297574,12.077971],[-11.456169,12.076834],[-11.513943,12.442988],[-11.467899,12.754519],[-11.553398,13.141214],[-11.927716,13.422075],[-12.124887,13.994727],[-12.17075,14.616834]]]}};

//Transform GeoJSON polygon into string polygon for API
//GeoJSON uses the format [Longitude, Latitude] so we must swap the indexes of the coordinates to match [Latitude, Longitude].
//Next, transform from arrays into one string.
const maliAsStringPolygon = maliAsGeoJSON.geometry.coordinates[0].map(x => [x[1], x[0]].join(',')).join(',');
console.log(maliAsStringPolygon); 


const base = 'https://image.maps.api.here.com/mia/1.6/region?'
const params = {
	a: maliAsStringPolygon,
  z: 3, //Zoom level
  app_id: 'devportal-demo-20180625',
  app_code: '9v2BkviRwi9Ot26kp2IysQ'
}

const url = base + Object.keys(params).map(key => key + '=' + params[key]).join('&');
document.querySelector('img').src = url;