You can use workflow to update this field
I know I am a bit late... did u already solved it by urself?
If not (and if I got it right u got the ID on the NewForm in List B):
U could set the Value (Example: 5) via JS in a textbox (that can be hidden somewhere on the form)
after that u could make a little JS function "if textbox isnt empty set the checkbox true" -- or you make a formula that is only activated when something is entered in the hidden textbox and then u could set the value with JS.
and u could link the 5 with a formula with a parselookup and a JS function to set the value.
Just an idea... bcs i dont know if u already solved it (so I didnt go into details).
@Aleximo thank you for the idea, will give it a try. I was able to partially " solve" it using JS, not to set the dropdown to the ID value but to only display that item in the dropdown. So user still has to click on the dropdown and select the only item displayed which ID is the value passed on.
If you only need the lookup to set itself maybe you could try the example in the following link:
https://community.nintex.com/t5/Community-blogs/Breaking-The-Rules-A-Dive-Into-The-Nintex-Forms-Rule-System/ba-p/78874
under -Examples- the 2nd one: Automatic Option Selection
