Hi there! Unfortunately, as you've noted, there's no IN keyword in Tableau. However, you have a few options.
First of all, you could just use the OR keyword, e.g.
columnName = "parameterValue1" OR columnName = "parameterValue2"
Alternatively, you could do a neat trick with CONTAINS(). Specifically, you would search for your columnName value within a concatenation of your two parameter values. Something like this:
CONTAINS("parameterValue1||parameterValue2||", [columnName] + "||")
The reason for the double pipes (or it could be any unusual combination of characters) is to make sure that you don't get an internal match (e.g. matching "meterVal").
I'm not entirely certain what you are working with, but I think it would be something like:
if parameter = 1
IF columnName = "parameterValue1" THEN 1 END
elseif parameter = 2
IF columnName = "parameterValue1" OR columnName = "parameterValue2" THEN 1 END
) = 1
There are a coupe of issues. First, Tableau doesn't allow Booleans to be returned inside an IF statement. Not sure why, but it is what it is. So, I simply embed additional IF statements to give a result of 1 if the condition is met. Then, I wrap the entire main IF statement in parentheses and compare the final result to 1 to get True or False result.
Also, I took the IN statement and turned it into an OR (or series of ORs if needed).
Hope that helps!