In D365 F&SCM, 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.