I am sitting here already awhile and can't find an answer on my own. 
I actually have pretty much the same Problem which was already asked (https://gis.stackexchange.com/questions/229161/multivalue-string-to-sql-expressions?newreg=696602b5989043bd96fee53f7127f486). 
But mine is more basic because of missing knowlegde. 
I tried to use the codechunk out of the other Problem but when i run it the expected Pick-List is always without Values. 
Some Basic Questions actually include missing comprehension of the functions. 

    multival = arcpy.GetParameterAsText(1)  # 'Store_1;Store_2;Store_3'
    vals = multival.split(";")

    where_stub = """"NAME" IN ({})"""
    stores = ','.join(["'{}'".format(x) for x in vals])
    where = where_stub.format(stores)

I actually don't know where the `multival`comes from and which Position its claimimg in the code. Maybe for a better understanding my task is to create a Route for a Pizza delivery with about 6 Stops. As data I got a file with 25 Stores which from I should pick various stores. To avoid a SQL-Expression I would like to have a Pick-List as in the other Problem shown (https://gis.stackexchange.com/questions/229161/multivalue-string-to-sql-expressions?newreg=696602b5989043bd96fee53f7127f486). 

My Code looks right now like this

    # Import arcpy module
    import arcpy
    from arcpy import env
     
    #workspace =  GetParameterAsText (0) ???
    arcpy.CheckOutExtension("Network")
    
    # Local variables:
    in_network_dataset = "Transportation/Streets_ND"
    outNALayerName = "BestRoute"
    

    # Process: Make Route Layer
    arcpy.MakeRouteLayer_na(in_network_dataset, outNALayerName, "TravelTime", 
    "FIND_BEST_ORDER", "PRESERVE_BOTH", "", "", "ALLOW_UTURNS", 
    "Oneway;RestrictedTurns", "", "", "", "")

    # Process: Select
    Stores = "Stores.shp"
    Stores_Select = "Stores_Select_Output"
    Expression = arcpy.GetParameterAsText(0)
    vals = Expression.split(";")

    where_stub = """"NAME" IN ({})"""
    stores = ','.join(["'{}'".format(x) for x in vals])
    Expression = where_stub.format(stores)

    
    arcpy.Select_analysis(Stores, Stores_Select, Expression)


It will execute until the select-process but then it does not make a difference between my script and a totaly empty one... 
Can Someone explain me how to get the Parameters into my Pick-List and maybe some Ground explanation so i understand what the Codechunk from (https://gis.stackexchange.com/questions/229161/multivalue-string-to-sql-expressions?newreg=696602b5989043bd96fee53f7127f486) includes?

**Edit:** 
This is how my Parameter Panel looks right now:
[![Pic1][1]][1]


  [1]: https://i.sstatic.net/z1ePy.png