First you need create a config file as ~/.ssh/config, then inside add code below:
# Structure example with Github
Host github.com
# UseKeyChain yes //Additional property
# AddKeysToAgent yes //Additional property
IdentityFile ~/.ssh/id_rsa.github
User git# Structure example with VSTS
Host visualstudio.com
IdentityFile ~/.ssh/id_rsa.vsts
User git