Conditional hidden field

It’d be great if you could conditionally fill in a hidden field, or otherwise allow the user to select if those conditions are not met.

Use case - we have different “tiers”, some which only allow a single project, other multiple. If they are on a single tier, I wish to prefill certain values as a hidden field. If they are on a multiple project tier, I want them to be able to select. Right now I have it as select, but only a single value available for some users, so not optimal UX.