Allow specific user to release their own quarantined messages

Create a new quarantine policy with the release option enabled and assign it to said users. The assignment part is the tricky one, as you have to do so via the corresponding anti-spam, anti-phish and additional policies, depending on which types of quarantine messages you want them to be able to release. For each of those, you will have to create a separate policy (i.e. a new anti-spam policy), assign the newly created Quarantine policy to the corresponding actions, then make sure the anti-spam policy only applies to the users in question. Rinse and repeat for any other policy types needed.

All the steps should be covered here: https://learn.microsoft.com/en-us/defender-office-365/quarantine-policies