Update field from second form based on Lookup value on first form.

Update field from second form based on Lookup value on first form.

I'm sure this has been asked a zillion times. I am using Lookup field to obtain a value from a second form. Based on the choice I want the corresponding value for another field on the second form to be input into the field on the first form. I know this is a set value action of some sort but am new to Zoho Deluge and cannot see how to do it or see any build functionality on the field properties to achieve this. Basis stuff but how?