Provider in ansible tower

ansible --version
ansible 2.2.1.0
config file = /etc/ansible/ansible.cfg
configured module search path = Default w/o overrides

I am using Tower to manage network devices, i have added the username and password as inventory variables to pass the credentials to provider.Could someone help how to pass provider in ansible tower other than inventory variable? I tried giving the username and password in the network credentials, But it tower doesn’t accept the value from network credentials.