For XML, "&" is a special character that "describes" other special characters - this si known as an "escape character" just as "\" is in a C# string.
To include an ampersand in your string, replace it with
"&"
:
<appSettings>
<add key="MongoCon" value="mongodb://asif:pass?@cluster0-shard-00-00.k6lme.mongodb.net:27017,cluster0-shard-00-01.k6lme.mongodb.net:27017,cluster0-shard-00-02.k6lme.mongodb.net:27017/myFirstDatabase?ssl=true&replicaSet=atlas-9yuzik-shard-0&authSource=admin&retryWrites=true&w=majority"/>
</appSettings>