A few people have been asking about this as they're working with IntelliGantt. SharePoint has a feature whereby you can 'disable' NTLM login and instead present a web form authentication challenge. Up to now IntelliGantt would alway use NTLM by default. However, with build 18.104.22.168 (which we just posted), IntelliGantt provides the ability to force a login when first connecting with a SharePoint site.
The built-in Windows Login form will still appear, but the value IntelliGantt will use will duplicate the same username/password values in the form authentication.
By default this 'Require Initial Login' is turned off. You turn it on by opening the 'SharePoint Settings' form. In the desktop this is found under the 'Settings' menu. In the Add In, it's also found under IntelliGantt->Settings. Because there are just a few way to customize SharePoint my hunch is you'll see more SharePoint Settings options in the future.
Here is the screen with the checkbox at the bottom. In case you're wondering what all the 'priority' and 'status' strings are for, this is how International users can customize the priority and status fields IntelliGantt uses when updating task values in SharePoint in your preferred language.