The purchase request form is where most requisition processes are won or lost, and the mistake is nearly always the same: asking finance's questions rather than the approver's. The person filling it in is not a finance professional, is doing it between other work, and will find another way to buy the thing if the form is tedious. Five fields answer the approval question; the rest can be added afterwards.
The five fields that matter
What is being bought, roughly what it will cost, which budget or cost centre carries it, when it is needed, and why. That is enough for a budget holder to decide. The why field is the one most often omitted and the one approvers read first, because it is the only field that explains the request rather than describing it.
What to leave off
Supplier selection, account codes, tax treatment and detailed specifications. Purchasing and finance can add those after approval, and asking a requester for them produces either blanks or confident wrong answers that somebody has to correct. A form that demands a general ledger code from a site manager is a form that will be filled in wrongly.
What the form should give back
An acknowledgement, a reference, and a way to see the status. Requests that vanish into silence are why people stop using the process. This is a small amount of work at implementation and it does more for adoption than any feature, because it removes the reason people chase.
Questions people ask about purchase request form
Should the form ask for a preferred supplier?
As an optional note, since the requester may know something useful. Making it mandatory bypasses purchasing's agreements, which is usually the opposite of what you want.
How do we handle requests with an unknown cost?
Ask for an estimate and a range. An approval given against an estimate should be revisited if the actual comes in materially higher, which is a rule worth writing down.
Should attachments be allowed?
Yes. A quote or a screenshot answers more approver questions than any number of extra fields.