In a URL, you can add ampersands (&'s) followed by parameters you wish to pass into a form (e.g., "http://www.site.com/db.nsf/8A8BB01ADEB05120852564180068A3F4?OpenForm¶m1=7¶m2=key1")
In the receiving form, create a variable called query_string. This field will be initialized by Domino to everything following the question mark in the URL (in the example above, this would be "OpenForm¶m1=7¶m2=key1"). In the default value formula for the field you want to check this field in, parse the value of query_string (e.g., to get the last parameter, use "@subset(@explode(query_string;"&");-1) " to get "param2=key1"). You can then do anything you want with the parameter.