See explanation below.
Go to Navigation pane > Modules > Organization administration > Number sequences > Number sequences.
Select Number sequence.
In the Number sequence code field, type seq.
In the Name field, type a value.
In the Segments section, click Add.
Select Constant and enter a value of seq
Click Add again.
Select Alphanumeric and enter a value of ######
In the General section, ensure that Continuous is set to Yes.
Under Number Allocation, enter 1 in the Smallest field.
Click Save to save the number sequence.
Reference:
https://docs.microsoft.com/en-us/dynamics365/fin-ops-core/fin-ops/organization-administration/tasks/set-up-num