Hello @OOlashyn,
No need to apologise, I think it's me that's making things unclear. Unfortunately, I'm still having problems, sorry.
I added the Account lookup field back onto the IP form and did not hide it.
I then went into the Portal Management model-driven app.
I found my Web Form Step for creating the IP record.
I clicked the Form Options tab.
I scrolled down to the Custom JavaScript section and adding my schema field name where you said I should add it. See below:

I then cleared the cache.
I went back to my portal.
Registered as a new user and created a Contact.
I then created an Account.
The other Web Form Steps adds more fields to the Account.
Then finally my last step is where I create the IP record.
When I look at the form, the Account field is still showing. I can't select it, but it's not hidden.
I complete the form.
I go back into the model-driven app, but unfortunately my IP record is still not associated to my Account record.
Any other thoughts please? No pressure if not. Potentially I must have set something up incorrectly previously, such as another value on the Web Form Step maybe?
Thanks for all your help,
Garry