First of all, if you are looking for an export of all records (or if you only have one archival institution in your AtoM site), then you can probably run the command-line export without needing to limit to a repository - just export everything, no SQL parameters needed. However, I'll include some tips below for you to take a look at the AtoM instance and find the repository slug if desired.
There are a number of ways you can get the slug value you need via AtoM.
Another name for repository records in AtoM is Archival institutions - so you can see all repository records that have been created in AtoM by navigating in the user interface to Browse > Archival institutions. If your users have modified the menus (so this link isn't available for example), you can also manually navigate to [your-site]/repository/browse. You can see an example of the repository browse page in our demo site, here:
If you click on a repository record, it will show a count of linked top-level descriptions in the holdings sidebar. This example in our demo site has 18 holdings:
Another way of checking this information: open the CSV export you created. There should be a repository column in the CSV. If this is populated with a name, then you will know the descriptions are linked to a repository record. You should then be able to search on the name in AtoM, find the repository record, and get the slug.
Yet another way: find one of the descriptions in AtoM's user interface that you know is part of the target export. On its view page, you will see a box in the upper left corner that will either have the name of the linked repository record, or else a repository logo if one has been uploaded. You can click on this to navigate to the associated repository record. Here's an example description linked to the Women's College Hospital Archives in our demo data:
Clicking on the repository / archival institution name in the box at the top left takes us to the repository record: