Log in to Sophos Central as a Super Admin (or with a role that has sufficient permissions).
From the menu: go to My Products → Global Settings → API Credentials Management (or Settings & Policies → API Credentials, depending on the portal version).
Click the Add Credential / Add button.
Enter a name, choose a role.
After creating, you will be shown the Client ID and Client Secret — the secret is shown only once, so make sure you save it.
Next, go to the Settings > Data Collection > API tab in Logsign USO, and enter the Client ID and Client Secret values you obtained.
After that, log extraction will begin, based on the log start date you specified.