Set Sqldatasource Parameter Value At Onclick Event Of Button

i have formview and gridview in onepage, and formview bind to one sqldatasource, which have select commandtype is storedprocedure,

now stored procedure have one optional parameter and select command.

 in gridview i have one button,now i want when user click on tha button that record's value comes in formview and its mode is edit.

now for that i have to pass that stored procedures parameter value,to stored procedure now how can i add the parameter value when the onclick event of gridview button is fire.

 thanks