First lets assume that i have only one dimension created with geomakepoint, and after that, keep in mind that i am using the default map visualization.
QlikSense automatically knows that the geomakepoint dimension is used to plot the points on the map, and the dimension that the user selects on the hover menu is used as label of the circle.
If i have two dimensions with geomakepoint, QlikSense needs to know wich one i want to use. So, after selecting the dimension that goes to the label, a submenu appears and the user informs the geomakepoint dimension that is going to be used to plot the points.
How can I achieve this behaviour in a extension that we are creating? Is there any example that we developers can use?
The Map chart appears to first make sure that your App has loaded columns that are tagged with either $geomultipolygon or $geopoint system tag. If you don't, you won't be able to pick a dimension. And if you drop a dimension onto the chart, you'll get the "The Action you are attempting cannot be applied to this visualization" message.
If you've satisfied that condition, then after you've chosen your dimension, if there are multiple columns tagged with those tags, you'll then get that sub-menu to which you refer.
So your extension would have to follow the same procedure as the built-in Map chart; detect columns tagged appropriately. If the system tags aren't appropriate for your extension, you can invent your own.