Access Smart form fields from Requisition Creation Page

Hi,

We have a requirement to prevent the end-user from submitting a requisition if the total requisition Amount (Quantity * Price) is more than 5000 K and no agreement selected .

As there is no available standard requisition Object under "Application Composer", so it wouldn't be applicable to do that using groovy script validation.
The other option we have we to disable the "Add to Cart" button if these conditions are satisfied.

I added a boolean expression that shows true or false based on the condition satisfaction and it works effectively in the form, however, while trying to access these fields(quantity,price,agreement) from the buttons toolbar, it looks like that this area of the page are not able to access the smart form content.