This works if you authenticate using a public/private key pair:
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Open Sourcetree and go to "Tools > Create or Import SSH Keys" (this will open a PuTTY Key Generator window),
Set the number of bits in a generated key
to 2048 and click on "Generate". -
Once generated go to git server panel and navigate to "My account > Profile > SSH Keys" and click "Add a public key".
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Copy the generated public key from the "PuTTY Key Generator" window ("Public key for pasting into OpenSSH authorized_keys file:") to git panel and save it.
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In the "PuTTY Key Generator" window enter a "Key passphrase" and "Confirm passphrase" and click "Save private key" and Public Key (for other uses too). Make sure to save it somewhere where you can find it again.