Scalar parameters (imports and exports) are always entered with a static value or can be assigned to an SSIS variable. In the case of exports, only variables are allowed because it wouldn’t make sense to enter an export in a static value.
As shown in the screenshot below, there are three import parameters. One is filled with a static value (SEARCH_CITY) and the other two (SEARCH_CUSTNO and SEARCH_NAME) will be filled with a variable. An @ must be added right before the name to mark a value as a variable.