Open with the problem you are solving, not your preferred technology. Who will use it day to day, how often, and what happens today? A vendor who knows what you are trying to achieve will suggest a cheaper route to it; someone handed only a list of screens can only price your assumptions along with the work.
Describe the scope as user stories livewire or react scenarios: who does what, and what happens next. Equally important, list what the first release deliberately excludes. A written out-of-scope list saves more argument during acceptance than any other single page. Indicate as well which items are decided and which may still change — honest teams price those differently, outsource software development and concealing the open questions helps no one.
Write down the hard constraints. These include the platforms and services involved, custom web portal development existing databases and their quality, security and compliance rules, expected load, which devices matter and stacks you cannot change. If there is a hard date, say why: a good team is usually able to cut the right scope to protect it, but only if they know it exists.
Define what completion means feature by feature. Acceptance criteria do not need any formal notation: a short paragraph describing the expected behaviour is enough. That one addition shortens the review at the end considerably and eliminates most late-stage disagreement.
One last thing, ask for a specific format. Ask for a task-level breakdown, the assumptions behind each number, the risks the team sees and a low number and affiliate platform development a high number. Read a wide range as useful information rather than evasion: it usually points to exactly which requirement is unclear. Then tighten that section and request a revised number — the revised figure is much more reliable.