Hi @Anonymous,
Yes, the LastSubmit property only applies to EditForm.
The EditForm has two mode, Edit Mode for record editing, and New Mode is used for creating new record.
I think here you are using the New mode, but it should be the EditForm control.
So if here you are trying to get the Server side ID once submitted by the EditForm control, this property would work.
For how EditForm works, please check:
Edit form and Display form controls in PowerApps
Further, as @Meneghino suggested, using patch () function would also work. The patch () function would return the record that has been created, including the Server side field.
Quoted:
"The return value of Patch is the record that you modified or created. If you created a record, the return value may include properties that the data source generated automatically."
Reference: Patch() function
Regards,
Michael