You should add an additional attribute to Customer called MarkerInfo. Now, whenever you save a customer, you should set the MarkerInfo attribute to something like this:
$Customer/Name + '|Address:' + $Customer/Address + '|Branche:' + $Customer/Branche
Thanks Eric! I will implement and test it when an other error is solved as well. See here: https://forum.mendix.com/link/questions/89175.
Hmm.. You are using a widget which is based on my GoogleMapsCustomMarker widget but not my widget. So, contact that developer ;)