If you have added a Custom Form action to your Workflow and it's not working as you expected, use these guidelines to troubleshoot it:
1) Login to the WebApp
2) Open a sales order and, before reaching the point in the workflow where a custom form* opens, open the DevTools (F12)
*Side note: You can also create a dummy/empty custom form to access it.
3) Go to the DevTool's "Network" tab and then continue the order's workflow to open the custom form.
4) Within the "Network" tab of the DevTools, search for a line that starts with "CForm" like this:
5) Right click the line and select "Open in new tab"
6) From the new tab, that'll display your custom form if you had something displayed in the code, open the DevTools (F12)
7) Within the new DevTools window open, you can now navigate to the Console and perform actions in real-time: