Defining a custom name joining certain previously defined strings like so:

```
df[:join([Str1,Str2,"_",Str3])] = zeros(length(df[:Str1]))

```

Returns the following error: 

```
MethodError: objects of type Symbol are not callable

```

I understand that `join` doesn't work like that. Is it possible to convert `join([Str1,Str2,"_",Str3])` to a variable?