LET LocationCode = input('Enter value for LocationCode', 'Input box');
SQL EXEC [ETTDS].[spLook] @parse_field='', @usid_facode='$(LocationCode)';
If you want to know what type your stored procedure needs, you should ask your friendly local database administrator.
'$(LocationCode)' will put the value of the variable LocationCode in single quotes, so it is treated as a string. If it must be a number then leaving out the quotes should do. Filling in a string instead of numbers will trip things up though. You could try a num#(LocationCode) first to try to make a number of whatever was input.
LET LocationCode = num#(input('Enter value for LocationCode', 'Input box'));
The reload/load script can't access or interact with any UI objects like charts and sheet input boxes. It can use the variables of the document.
The input boxes you can use on sheets are for allowing the user to easily change values of variables which can then be used for example to change selections or change a calculation.