There is an updated version of this post with new instructions now here:
created some chunks in category with id 11 with some text and an additional placeholder
default-value: @EVAL return $modx->runSnippet('migxChunklistToJson',array('category'=>'11'));
{"formtabs":"[{\"MIGX_id\":\"1\",\"caption\":\"chunktest\",\"fields\":\"[{\\\"MIGX_id\\\":\\\"1\\\",\\\"field\\\":\\\"name\\\",\\\"caption\\\":\\\"Chunkname\\\",\\\"inputTV\\\":\\\"\\\",\\\"inputTVtype\\\":\\\"\\\",\\\"configs\\\":\\\"\\\",\\\"sourceFrom\\\":\\\"config\\\",\\\"sources\\\":\\\"\\\",\\\"inputOptionValues\\\":\\\"\\\",\\\"default\\\":\\\"\\\"},{\\\"MIGX_id\\\":\\\"2\\\",\\\"field\\\":\\\"extra\\\",\\\"caption\\\":\\\"Extra\\\",\\\"inputTV\\\":\\\"\\\",\\\"inputTVtype\\\":\\\"\\\",\\\"configs\\\":\\\"\\\",\\\"sourceFrom\\\":\\\"config\\\",\\\"sources\\\":\\\"\\\",\\\"inputOptionValues\\\":\\\"\\\",\\\"default\\\":\\\"\\\"},{\\\"MIGX_id\\\":\\\"3\\\",\\\"field\\\":\\\"selected\\\",\\\"caption\\\":\\\"\\\",\\\"inputTV\\\":\\\"\\\",\\\"inputTVtype\\\":\\\"hidden\\\",\\\"configs\\\":\\\"\\\",\\\"sourceFrom\\\":\\\"config\\\",\\\"sources\\\":\\\"\\\",\\\"inputOptionValues\\\":\\\"yes==1||no==0\\\",\\\"default\\\":\\\"\\\"},{\\\"MIGX_id\\\":\\\"4\\\",\\\"field\\\":\\\"selected_ro\\\",\\\"caption\\\":\\\"\\\",\\\"inputTV\\\":\\\"\\\",\\\"inputTVtype\\\":\\\"hidden\\\",\\\"configs\\\":\\\"\\\",\\\"sourceFrom\\\":\\\"config\\\",\\\"sources\\\":\\\"\\\",\\\"inputOptionValues\\\":\\\"\\\",\\\"default\\\":\\\"\\\"}]\"}]","contextmenus":"","actionbuttons":"","columnbuttons":"","filters":"","extended":{"migx_add":"","formcaption":"","win_id":"chunklist","multiple_formtabs":"","extrahandlers":"this.handleColumnSwitch","packageName":"","classname":"","task":"","getlistsort":"","getlistsortdir":"","use_custom_prefix":"0","prefix":"","grid":"","gridload_mode":"1","check_resid":"1","check_resid_TV":"","join_alias":"","getlistwhere":"","joins":"","cmpmaincaption":"","cmptabcaption":"","cmptabdescription":"","cmptabcontroller":""},"columns":"[{\"MIGX_id\":\"1\",\"header\":\"Chunk\",\"dataIndex\":\"name\",\"width\":\"10\",\"sortable\":\"false\",\"show_in_grid\":\"1\",\"renderer\":\"\"},{\"MIGX_id\":\"2\",\"header\":\"Selected\",\"dataIndex\":\"selected\",\"width\":\"10\",\"sortable\":\"true\",\"show_in_grid\":\"1\",\"renderer\":\"this.renderSwitchStatusOptions\",\"renderoptions\":\"[{\\\"MIGX_id\\\":\\\"1\\\",\\\"name\\\":\\\"selected\\\",\\\"value\\\":\\\"1\\\",\\\"clickaction\\\":\\\"switchOption\\\",\\\"handler\\\":\\\"\\\",\\\"image\\\":\\\"assets\\\\\\\/components\\\\\\\/migx\\\\\\\/style\\\\\\\/images\\\\\\\/tick.png\\\"},{\\\"MIGX_id\\\":\\\"2\\\",\\\"name\\\":\\\"unselected\\\",\\\"value\\\":\\\"0\\\",\\\"clickaction\\\":\\\"switchOption\\\",\\\"handler\\\":\\\"\\\",\\\"image\\\":\\\"assets\\\\\\\/components\\\\\\\/migx\\\\\\\/style\\\\\\\/images\\\\\\\/cross.png\\\"}]\"}]"}