Multi-Value parameters are an option. You do not have to use a list with them although they are not very intuitive.

I’ve created a simple example with a list of country names and codes (e.g. [Country Code] = “IT” and [Country] = “Italy”)

A created a multi-value parameter called CountryCode but I did not set the available values.

I added a datset that looked like this

SELECT 
*
FROM myCountryTable
WHERE [Country Code] in (@CountryCode)

When I run the report, I get a blank multi-value parameter field. When you click this field you see an empty list but you can simply type values hitting Enter between each to start a new line.

Once all the values had been entered, I just hit “View Report” and got the following (I’ve included the parameter as I was editing it for clarity

The advantage of this approach is that the report can handle as few or as many values as you like.