Setting missing field to default value based on a master template

Setting missing field to default value based on a master template

I will put my example in non elasticsearh language.

I have two tables table_a and table_b
table_a
Field1 Field2

table_b
field, field3

How do I make it such that after receiving data from Logstash, I can receive the following
table_c
Field1 Field2, Field3
Field 2 should contain a null or empty value

I heard that I can use a template to ensure non-existing field to be created in elsticsearch as field with empty value. Should I do this? How do I do this?