In D365 FO, when you create a fixed asset, the fixed asset number is generated based on the number sequence code of the selected fixed asset group.

In Data entry workflow, you can setup a data entry workflow template to create a fixed asset. For the first data entry step in this template, at least, select the Fixed asset group field and the Number field.

When a data entry workflow is started, on data entry, when you select the fixed asset group, automatically the number is generated.