To achieve this you will need to loop through all your polygons and do an intersection test with your proximity circle polygon. Since you are using the Bing Maps V8 web SDK, you can use the spatial math module to do the intersection test. Here is a quick example.

```
Microsoft.Maps.loadModule('Microsoft.Maps.SpatialMath', function () {
//Assume this is an array of polygons.
var polygons = [];
//Assume this is the polygon area you want to search in (i.e. circle);
var searchPolyogn = {};
//Loop through each polygon.
for(var i=0,len=polygons.length; i < len; i++){
//Test each polygon to see if it itersects the search polygon.
if(Microsoft.Maps.SpatialMath.Geometry.intersects(polygons[i], searchPolyogn)){
//If it does, do something, like change it's color.
polygons[i].setOptions({fillColor: 'red' });
} else {
//It didn't intersect, maybe change the color back to a default color.
polygons[i].setOptions({fillColor: 'blue' });
}
}
});
```

Here is a similar example: https://samples.bingmapsportal.com/?sample=select-data-in-drawn-polygon-area