If you already have your crystal report object set up with the report that you are going to run, all you have to do is something similar to below:

tcParameterName = the name of the parameter you want to set.
tuValue = the value you want to set the parameter to.
This.oReport = your crystal report object.

FOR EACH loParamDef IN This.oReport.ParameterFields
			WITH loParamDef
				lcParmName = LOWER(.ParameterFieldName)
				IF lcParmName = "@" + LOWER(tcParameterName) ;
					OR lcParmName = LOWER(tcParameterName)

					.SetCurrentValue(tuValue)
					RETURN .T.
				ENDIF
			ENDWITH
		NEXT loParamDef