You can do it more easily by using one of the overloaded methods of TFDConnection. There is one where the user name and password are supplied as string arguments so you don’t have to worry about underscores etc in the connection string and instead just let FireDAC take care of it for you.
Here’s an example from a working app. In my example the server and database are configurable so they are passed in as arguments - you could, of course put hardcoded values in there instead or make the whole thing configurable.