In order to see custom data tracked with Full Picture, you need to register them in Google Analytics as either custom dimensions or metrics.
In order to do that:
1. copy the highlighted parameter name that is under the setting field that you enabled in Full Picture.
2. Go to your websites Google Analytics 4 panel
3. In the left menu click “Custom definitions”
4. Choose either “Custom dimensions” or “Custom metrics” from the top of the table
5. Click a “Create custom dimention” or “Create a custom metric” button in the top right coner of the page.
6. Write a descriptive name and enter the parameter name in marked fields