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