Preseed Disk partition - Use existing partition with out create new partition - Patman ubuntu ISO

I have working on the Ubuntu upgrade from 14 to 18.04 ISO. So in my system already having two partitions:

/dev/sda1  Ext4   /
/dev/sda5  swap

I want to be use existing partition (sda1) to install the latest OS 18 without format (format: no, keep existing data) and assign this as mount as root (mount: /)

I need to create the entries in to preseed file as like below configuration (manually select)

Use as : Ext4 jounaling filesystem
Format the partition: no, keep existing data
Mount point: /
Bootable flag: on

Please help on this. i have spent too much time for this.

Note : i can achieve while doing manually select, but i have to done on the unattened ubuntu ISO part.

Thanks,

Ramesh