Maxwell,

Parameters are passed by referencing the stored procedure parameter name in the EXEC statement in the custom query.  In the example below I have a stored procedure named 'GetApplicationNumber' that has a single varchar(50) parameter named '@guid' and I am passing the value from a forms token to the stored procedure;

Note: The naming syntax for the Parameter Names in the custom query depend on how you configured the connection to the database.  In the example above I am using a direct SQL connection so my parameter is named, versus an ODBC connection where the parameter name would just be a '?'. 

See the help files here https://www.laserfiche.com/support/webhelp/Laserfiche/10/en-US/administration/#../Subsystems/LFWorkflow/Content/Resources%2FActivities%2FCustom%20Query.htm for additional information on custom query parameter naming syntax.