Django - Retrieve or get data from selected foreign key field

Question

Relatively new to Django, I'm working on a Django project and attempting to retrieve particular foreign key object into variable when it's selected in Form.

model.py

class item_category(models.Model):
    idItemCat = models.CharField(primary_key=True max_length=5)
    nameCategory = models.CharField(max_length=150)

    def __str__(self):
        return self.nameCategory

class item_code(models.Model):
    idItemCat = models.ForeignKey(item_category, on_delete=models.DO_NOTHING)
    idItemCode = models.CharField(primary_key=True, editable=False, max_length=20)

    def __str__(self):
        return self.idItemCode 

I know I could retrieve object with making QuerySet such as .objects.last() and .objects.filter() or else, but it's just retrieve objects from database or existing data. What I'm about to do is, when a user submit a new data it'll retrieve particular foreign key object based on what I'm selected in this Form, so I could put into variable.

Any idea how should do it? it would be so much appreciated.