Unfortunately, it's very common for web developers who write scripts for submission forms to send email with the SMTP envelope "MAIL FROM" address set to the address provided by the user who submitted the form. They do this so that the recipient of the email can simply hit "Reply" to send an email to the submitter.
But I'm afraid that this is a bad idea on a number of levels.
By essentially forging the sender of the message, the emails will look spammy to pretty much any spam filtering service or software. Emails sent this way will violate SPF, DKIM, ADSP, and DMARC protocols.
There's no way to whitelist the sender of the message, since it's being set differently for every message.
What you can do:
Speak to your web developer and ask them to make a small change on the script:
1. Have them set the SMTP Envelope MAIL FROM and the header "From:" addresses to use a standard address that's within your own domain. You can then whitelist that address, so it will always arrive, no matter how spammy it might appear to our filters.
2. Ask them to set a "Reply-To:" header with the submitter's provided email address. All mail clients will recognize this, and when the recipient replies to the message, they will send to that specified address.
Another option, if you absolutely cannot change the form submission script, is to have the emails sent to a special address on your end, and to disable spam filtering for that account or to set it to a higher spam quarantine cutoff number.