I don't come here very often so forgive the delay in responding.
I only started using AntForm yesterday but I think you have to launch your build.xml _first_ and then have the build.xml invoke your AntForm or AntMenu tasks. That's how I did it and it works quite well.
If that is not satisfactory and you need really to invoke the build.xml from an AntForm, you _may_ be able to do it by using the AntForm API. For example, if you write a
Java program, I think it can probably display AntForms and those AntForms _may_ be able to launch a build.xml but I'm not really sure. There is a page on using the AnfForm API in the AntForm documentation that comes with AntForm.