Sometimes I've thrown in a button that'll launch the external program. Mostly I've just left it to the browser to turn around & forward the link to the chosen application.

Thank you Mike for the answer.
I usually use the second way but it isn't too smart- the browser page remains opened or sametimes browser loses association.
In the meantime I tried create script to run external application (ProjectWise) but I can't get hotspot content value and transfer its to the program.
There is nor @function nor lotus script instruction to get hotspot properties.

An application registers itself in the Windows registry, so you can look it up from there. Then launch it with the URI as an argument, it's supposed to work from there.

https://stackoverflow.com/questions/389204/how-do-i-create-my-own-url-protocol-e-g-so

Does Projectwise Explorer set a file association (extension .xml to be opened by Projectwise Explorer) when the program is installed?  If you right-click on a file in Windows Explorer with extension name 'x', can you choose Projectwise Explorer as the application to open files of this type with? I'm just trying to understand if there is no default file association with Projectwise Explorer or if the protocol can't be added to the hotspot.  I can associate a hotspot with ldap:// and ftp://, so at least some protocols other than http://, https:// and notes:// are supported.  

ProjectWise (PW) doesn't have files and special files extensions.
Windows and programs like Thunderbird  recognize PW links because PW's links begin with pw://....
I can tell Windows or Thunderbird that protocol pw:// requires running PW Explorer and they can launch it and then PW Explorer open internal document.
But Notes can't launch PW Explorer. If you know how associate hotspot protocols (eg. pw://)  with application (c:/Program Files(86)/.... PWC.EXE)  in Notes it will be ok.

Finally I wrote agent which launches the Projectwise program.
Because there is no information about hotspot properties in the notes document
I used export document to XML and then parse XML to get information about hotspot value.