You only have to start Pageant once (per Windows login) to make it available for all apps including SourceTree - what I tend to do is add Pageant to my Startup folder in Windows and give it a parameter of the .ppk key, that way when I log in I have to type the passphrase and then it's available for all apps.
However alternatively if you set the key in Tools > Options > SSH Client inside SourceTree and check the box 'Automatically Start SSH Agent' then if Pageant isn't already running when SourceTree starts, it will start it for you and load this key. You will still need to type the passphrase, this is a security measure which is safeguarding your key from other people should they somehow get hold of it.
Supported Platforms macOS Sourcetree has a lot to offer and, like many developer tools, finding and using it all can be a challenge, especially for a new user. Everyone might not love ...
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