I tried to deeplink into the Venmo app, but couldn't find a lot of public documentation on it. So I cracked the main app, and found all of the deeplinking strings, which are listed below:
Now, you can't see the parameters that they take, but some of them are relatively straightforward. Additionally, they serve as a launching off point for searches on GitHub.
For example, the main one I was looking for was this one:
which I found in a Venmo sample application.
That example will open up a transaction to pay me for $10, with the note "Note". Changing the "txn" to "charge" will change it to a request.