How do I set user field in form to the currently logged in user?

Remove user field from rendered form (using exclude or fields, https://docs.djangoproject.com/en/dev/topics/forms/modelforms/#selecting-the-fields-to-use )

class CandidateForm(forms.ModelForm):
    class Meta:
        model = Candidate
        exclude = ["user"]

Find user profile and set user field in the create view.

class CandidateCreateView(CreateView):
    ...
    def form_valid(self, form):
        candidate = form.save(commit=False)
        candidate.user = UserProfile.objects.get(user=self.request.user)  # use your own profile here
        candidate.save()
        return HttpResponseRedirect(self.get_success_url())