By using Create Releases
The only way to automatically get the Blanket Agreement number on the Release Header is by creating the releases from the Blanket Sales Agreement screen by clicking the Create Releases button.
If you do not want to use the "Create Releases" functionality from the Blanket, then you would need to manually enter the Blanket Number on the Sales Order Header.
Can create a Sales Order Line Defaulting Rule to default the line Agreement from the header agreement. With this, the Blanket Line Number field will be automatically populated based on the correlation between the item selected in the sales order, and the line on which that item appears in the blanket sales agreement.
Use following setup for defaulting of agreement number to order line.
Order Management responsibility: Setup > Rules > Defaulting> for Order Line entity, attribute "Sales Agreement Number", define the defaulting rule as follows:
Defaulting Condition : Always
Defaulting Source Rule : Related Record
Default Source/Value : Order Header.Sales Agreement Number