You can use the “Default value” setting to preselect an option from the relationship dropdown. You will need to create Special Records in advance, one for each option you might want to use as a default.
If you don’t want the user to see the dropdown at all you can also set the Wrapper to “Hidden row”, so it won’t be visible on the form but will still submit the value.
There are likely other ways to also achieve this too, you could set the relationship using a rule after submission for example, it all depends on exact needs and config.