Hi @felicini
The way lookups work in SharePoint is that you reference the lookup field to the ID of the field of the other list. So let us assume that I have a list Course and this has a column Department. I have another List Person and here I have to lookup to the course based on the department. So in the person, the column department will refer to the item ID of the department in the List Course.
In forms, you just get the label and so to refer that to lookup you first have to get the relevant item.
Please see the screenshot below and replicate:

Here, Get Items action will just return one item as my department is unique value.
Hope this Helps!
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