sometimes here on the community someone ask how to set through a workflow a lookup field, so I've decided to write here how to do that..it's very simple!
In your workflow you need to know what is the ID of the elements in the list that's connected through the lookup column.. if your list has a lookup on the following list:
and you want to set on your item a lookup to Item1, then in your workflow you have to set that field to the corresponding ID
but..what if you want to update a lookup column with multiple values?
Firstly, check that the lookup column allows multiple values..you have to explicit set it and sometimes simple things may be forgotten..
Then you have to create a string with all the IDs of the corrisponding elements, separated by ;#;#, so if in our case, we want to create a lookup to Item1 and Item3 we have to set the field as in the image
That's all..Easy, isn't it?