You can generate "campaigns".
In App Store Connect, go to "App Analytics", choose the app, then "Acquisition", then "Campaigns". Click on (+).
This will generate a "campaign link" which you can give to your partner web sites, and you'll be able to see what proportion of sales come via that link.
Note I've never used this. I don't know if it is useful for paying commission to those partner web sites, or whether it's only useful for approximately measuring the effectiveness of a campaign. In particular, you need to check if Apple filter the data for privacy: a lot of features in App Analytics rely on the users opting in, and only a minority do so, and then Apple hide some stats if the numbers of users is below a threshold.
Note if you do this, make sure that the partner web sites indicate somewhere that they are affiliates who get commission. This is a legal requirement in some places.
(Edited to add: note that even without this, the Analytics page can show referring web sites and apps; I see for example that a small proportion of my sales come from Google searches, some from links shared on Facebook etc. The advantage of the "campaign" links is that if someone copies&pastes the link from your affiliate web site to e.g. an email, then you see the original affiliate as the source of the sale, not just "email".)