Find your organization in the top left. Click the organization name, then Tools & settings > Organization settings.
Click Integrations > Installed apps.
Find and click the Glean app you created earlier.
In the top right, click Manage > Approve for organization. Approve the scopes.
In the top right, click Manage > Add to workspaces. Select all workspaces by clicking the top-most checkbox. Also select Default for future workspaces.