Populate certain field when Creating a record from subgrid.

Hi All,

I have an Entity A and Entity B. Entity B has a sub grid on Entity A.

i wanted to try out a functionality in which if I am creating a new Entity B record from sub grid(clicking on Add record + button), i want certain field(Single Line of Text) to have "Not required" populated. 

whereas when i am navigating to the entity from sitemap and creating a record, the field should not be automatically populated.

Is it possible? I am trying out JavaScript for this.

Can you please suggest/help on how can I achieve this?

Thanks in advance.