When you select the menu option Setup -> Source Controls you will see the currently defined source control connection records. These allow OneScript Client to work with you source control. When you have one of the items selected in the list you can click the button "Local Credentials" to add your local credentials. You do not need to do that when using Windows authentication.
Adding or editing source control connection information is saved on the server and is usable by all OneScript Clients that connect to that OneScript server. For the "Repository Path or URL" only provide the URL to the root of your source control repository and not the full path to the project you intend on using. This was the source control connection information can be used for multiple projects that have different parent paths.