As of 21/2/2018 you can now use metadata to set a lookup value on save/submission of a web form step. This is as per release 188.8.131.525. You can find more info here.
It’s very simple stuff. Create a web form metadata record.
Type = Attribute
Attribute Logical Name = The lookup you want to set the value of
Set Value On Save = Yes
Type = Value and Value = The Guid of the record on the other end of the N:1 relationship.
Below is my example from the video where I populated a look up on case where the primary entity was webform, allowing us to know which web form created which case.
If you have any feedback or questions please don’t hesitate to reach out via comments below or LinkedIn.