How to fill a field depending of what was choosed on previous field?

How to fill a field depending of what was choosed on previous field?

Hello friends!

Someone knows, please, how can I automatically fill a field based on the choose I made on the previous field?

For example, I have a form which first field makes a "lookup" on another form. I want to fill the next field with the first field of that another form, based on the choose I made here. Is this possible, please?

Thank you again. :)