Create the map geofield behavior.

modules/core/map/config/install/farm_map.map_behavior.geofield.yml

@@ -0,0 +1,11 @@
+langcode: en
+status: true
+dependencies:
+  enforced:
+    module:
+      - farm_map
+id: geofield
+label: Geofield
+description: 'Copies map layer data into a form textarea.'
+library: 'farm_map/behavior_geofield'
+settings: { }

modules/core/map/farm_map.libraries.yml

@@ -25,3 +25,8 @@ behavior_wkt:
   dependencies:
     - core/drupalSettings
     - farm_map/farm_map
+behavior_geofield:
+  js:
+    js/farmOS.map.behaviors.geofield.js: { }
+  dependencies:
+    - farm_map/farm_map

modules/core/map/js/farmOS.map.behaviors.geofield.js

@@ -0,0 +1,13 @@
+(function () {
+  farmOS.map.behaviors.geofield = {
+    attach: function (instance) {
+      instance.edit.wktOn('featurechange', function(wkt) {
+        console.log('here!');
+        document.querySelector('#' + instance.target).parentElement.querySelector('textarea').value = wkt;
+      });
+    },
+
+    // Make sure this runs after farmOS.map.behaviors.wkt.
+    weight: 101,
+  };
+}());