diff --git a/tests/php/Extensions/AjaxFieldTest.php b/tests/php/Extensions/AjaxFieldTest.php
new file mode 100644
index 0000000..74cf75e
--- /dev/null
+++ b/tests/php/Extensions/AjaxFieldTest.php
@@ -0,0 +1,88 @@
+<?php
+
+namespace SilverStripe\AnyField\Tests\Extensions;
+
+use SilverStripe\Admin\LeftAndMain;
+use SilverStripe\Forms\TreeDropdownField;
+use SilverStripe\CMS\Model\SiteTree;
+use SilverStripe\Dev\SapphireTest;
+use SilverStripe\AnyField\Extensions\AjaxField;
+use SilverStripe\CMS\Forms\AnchorSelectorField;
+use SilverStripe\Control\Controller;
+use SilverStripe\Control\Director;
+use SilverStripe\Control\HTTPRequest;
+use SilverStripe\Control\Session;
+use SilverStripe\Core\Injector\Injector;
+use SilverStripe\Forms\Form;
+use SilverStripe\Forms\FieldList;
+use SilverStripe\LinkField\Models\SiteTreeLink;
+
+class AjaxFieldTest extends SapphireTest
+{
+
+    protected $use_database = false;
+
+    public function testExtensionApplied()
+    {
+        $field = TreeDropdownField::create('Name', 'Label', SiteTree::class);
+        $this->assertTrue($field->hasExtension(AjaxField::class), 'AjaxField is applied to TreeDropdownField');
+
+        $field = AnchorSelectorField::create('Name');
+        $this->assertTrue($field->hasExtension(AjaxField::class), 'AjaxField is applied to TreeDropdownField');
+    }
+
+    public function testUpdateLinkModalForm()
+    {
+        $request = new HTTPRequest(
+            'GET',
+            'admin/methodSchema/Modals/AnyFieldForm',
+            ['key' => SiteTreeLink::class],
+            []
+        );
+        $request->setSession(new Session([]));
+        $leftAndMain = LeftAndMain::create();
+        $controller = $leftAndMain->Modals();
+        $leftAndMain->setRequest($request);
+
+        $field = TreeDropdownField::create('Name', 'Label', SiteTree::class);
+
+        Form::create(
+            $controller,
+            'Modals/AnyFieldForm',
+            FieldList::create($field)
+        );
+
+        $this->assertEquals(
+            'admin/Modals/Modals/AnyFieldForm/field/Name?key=' . urlencode(SiteTreeLink::class),
+            $field->Link(),
+            'Link is updated with the key'
+        );
+    }
+
+    public function testDontUpdateLinkModalForm()
+    {
+        $request = new HTTPRequest(
+            'GET',
+            'admin',
+            ['key' => SiteTreeLink::class],
+            []
+        );
+        $request->setSession(new Session([]));
+        $leftAndMain = LeftAndMain::create();
+        $leftAndMain->setRequest($request);
+
+        $field = TreeDropdownField::create('Name', 'Label', SiteTree::class);
+
+        Form::create(
+            $leftAndMain,
+            'LeftAndMain',
+            FieldList::create($field)
+        );
+
+        $this->assertEquals(
+            'admin/LeftAndMain/field/Name',
+            $field->Link(),
+            'Link is updated with the key'
+        );
+    }
+}