First thing first, please give a name to your field in Field Name (I love it when I write transcendent comments like this). Then, let's talk a second about the Field ID. This ID is used by Looker Studio to understand that a field is shared across data sources.
What does it mean? If you have a country field in a GA4 data source, and another country field in a Search Console data source with a different ID, creating a control on one won't filter anything on the dataviz based on the other. Because for Looker Studio, they are different fields. How to overcome this issue? Please refer to this article I wrote about How to build a control that filters across several data sources in Looker Studio?
For the moment, keep it as is, but in the near future I'll bet you want to use meaningful IDs to share them across data sources and build nice controls. Plus, once the field is created, you can't modify the ID, so think first!
Then, we're getting to the formula. No need to start with an equal sign, just drop your formula. Remember that you can't mismatch dimensions & metrics, so if your formula is based on a dimension, then only dimensions are accepted. If based on a metric, then only metrics are accepted.
Your formula can either be numeric (see below), or based on dimensions using LIKE REGEX... Please refer to this article on How to build a field using a CASE WHEN statement in Looker Studio? to learn more.