SFDCWallah

//getting community name from custom label
String communityName=Label.Community_Name;

//fetching id from network to get url
Network myNetwork = [SELECT Id FROM Network WHERE Name=:communityName];

//Fetch communityUrl from Network.getLoginUrl(myNetwork.id)using network id
String url=Network.getLoginUrl(myNetwork.id).substringBeforeLast('/');

//Final Community Url
String commurl=url+'/CommunitiesVfPage?oppId='+opplist[0].id;

//creating link of community url
String communityUrl = '<a href='+commurl+'> click here </a>';