This feature is available only in the Premium pricing plan.
Only Admins can update the numbering sequences.
Similar to setting the Custom Invoice Number Format, you can now set up a General Number Scheme for other items as well.
To do so, head over to Settings - General:

And navigate to the Numbering scheme menu where you can set custom sequences for projects, budgets, and tasks:

To see which variables are supported when creating a custom sequence, hover over the ℹ️ icon next to each item:

You can also use standard text along with the predefined variables.
For example, Task-{n}, Project-{yy}, etc.
Please note that there is no option for setting up a numbering sequence that will follow the number of your choice, i.e. there is no option that a sequence starts from, for example, 123.
💡 A task sequence cannot contain the 'company_code' variable