To access any external function library in XForms you need to provide a reference to a namespace that identifies the library, and you need to indicate the functions you want to access in the functions attribute on the model. So the first step is to add the following namespace to the top of the document:
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:sb="urn:formsplayer.com/softbars"
>
<head>
Now add a model which will contain the data about the soft-bar we will be installing, and reference the functions that will be called to do the adding and removing:
<title>Soft-bar Installer</title>
<xf:model id="m-soft-bars" functions="sb:GenerateBar sb:RemoveBar">
...
</xf:model>
</head>