Custom Form allows you to use your own page as an Action Launch Form, meaning that you can implement your custom layout and logic.

When defining this type of ALF, we have to provide a URL address of the custom page.

The main parameters will be passed to the page in the URL; for instance, if we have a MyForm.aspx page in the C:\\(...)\Web Server Extensions\15\templates\layouts folder, a sample URL will look like:
http://{server}/MyForm.aspx?List=value1&WorkflowID=value2&ItemID=value3&ActionName=value4&SourceURL=value5&RedirectURL=value6

Here is the list of the parameters:

• List – the list’s GUID,
• WorkflowID – the workflow instance’s ID,
• ItemID – the list item’s ID,
• ActionName – the action’s internal name,
• SourceURL – URL address of the source page,
• RedirectURL – URL address of the page to which users should be redirected.

To programmatically launch an action you can use the appropriate method of WBInterface. More information can be found in Datapolis Process System 2013 SDK.