Basically, an LBF is used in the portal by invoking it from within a portal template. So you have to build a portal template which contains the tag to call the LBF.
A forum post where the dev talks about implementing LBFs in the portal templates:
More on the current and future use of LBFs in portal forms
A couple specific techniques to use in LBFs for the portal
Then some items in the OpenEMR wiki, which is a site not widely known here in the forum. It can be difficult to navigate untill you get used to it but it does have a lot of good information in it.
