To update the values of custom fields, use the DB Field Name value in your Settings > Custom Fields for the field you want to update.

If you don't see that column, be sure to use the column selector to un-hide it:

You can do a checkout on creation if you add one of the following fields: assigned_user, assigned_asset, or assigned_location. This should be a valid primary key of the user, asset or location you wish to checkout to.