FileMaker and 4th Dimension both have a similar requirement that a user needs the ability to launch either application and log into a specific database.
While FileMaker’s approach has been to create a URL protocol for this, 4th Dimension has created a registered file suffix, .4dlink, that allows you to create a simple custom XML file that a user can double click on to launch the database application.
When the user double clicks on a .4dlink file, the operating system launches 4th Dimension and 4D parses the xml file to get the server address. You can also embed a user name and password in the file if you would like to eliminate the need to enter those each time you log in.
4D has a good tech note on the .4dlink file that also includes a simple database that allows you to enter your database information and have it generate a .4dlink file for you.