Unpredictable random number unique in table in MariaDD (not just incremental)

I have table of Users in MariaDB and I need to generate serviceNumber for each newly created user.

Example of this code: 865165

Only two requirements are:

• Unique in User table
• Unpredictable when creating user (Not based on AutoIncrement maybe?)

Is this possible with just database? Or I need to implement it in backend when creating user.