https://bootpanic.com/auto-storing-server-host-key-in-cache-with-plink/

Try prepending your script with:

echo y | plink -ssh root@REMOTE_IP_HERE "exit" 

This will pipe the y character through stdin to plink when you get the Store key in cache? (y/n) prompt, allowing all further plink commands to pass through without the need of user input. The exit command will close the SSH session after it has been established, allowing the following plink commands to run.

Here’s an example script which writes the external server’s Unix time to a local file:

echo y | plink -ssh root@REMOTE_IP_HERE "exit"
plink -ssh root@REMOTE_IP_HERE "date -t" > remote_time.tmp