For automatic icons there is no custom field selection, it’s based on taxonomies.
However, for automatic map generation, you can pick a custom field for the icon. Go to the ‘geocoding fields’ section in the MapPress settings. Where you have set ‘address line 1’, you can also set a custom field for ‘icon’.
To regenerate those automatic maps (for example, if you change the icon field), you can re-publish the posts. From the WordPress posts section use ‘bulk actions’ and then ‘edit’, and then ‘publish’.