How to setup TMG proxy for AppStore

Good day

My IS admin is having trouble setting the TMG 2010 proxy so that my MacBook Pro can connect to Mac AppStore as well as iTunes machine specs are :

MacBook Pro 13-inch (Early 2011)

Model Identifier: MacBookPro8,1

Processor :Intel Core i7 2,7 GHz

Memory: 4 GB

Operating System : Yosemite 10.10.3

Storage : 500GB

When we monitor the connection of the macbook through TMG it shows that HTTP is refusing and blocking the connection but HTTPS allows the connection.