gistfile1.txt

    public function safeUp()
    {

        $fieldGroupName = 'myFieldGroup';
        $fieldService = Craft::$app->getFields();
        $fieldGroup = new FieldGroup();

        $fieldGroup->name = $fieldGroupName;

        $fieldGroupSuccess = $fieldService->saveGroup($fieldGroup);


        if ($fieldGroupSuccess) {
            $fieldGroupId = null;
            
            // FIXME: Couldn't find a method to search for a field group by name
            $fieldGroups = $fieldService->getAllGroups();
            foreach ($fieldGroups as $_group) {
                if ($_group->name == $fieldGroupName) {
                    $fieldGroupId = $_group->id;
                }
            }
            if (!$fieldGroupId) {
                echo "Couldn't find ID for field group, aborting";
                return false;
            }
            $field = $fieldService->createField([
                'type' => 'craft\fields\PlainText',
                'groupId' => $fieldGroupId,
                'name' => 'fieldName',
                'handle' => 'fieldNameHandle'
            ]);
        }
        return $fieldService->saveField($field);
    }