How do I add additional 50 GB?

If you are asking how to add 50GB of storage directly to your phone, you can't. Once purchased, your phones memory was fixed and can never be increased.

If you are asking how to buy more iCloud storage, so you can sync more data with the cloud, this link will help with that --> Upgrade your iCloud storage plan - Apple Support

Again, this will NOT in any way add memory to your iPhone. The only way to get more memory on a phone which is low on memory, is to delete things off your phone, such as Apps, Photos, Music, Video, Messages, Mail, etc.

To your phone, you don't.