@Ulysse4733 yes that's right, the JSON goes into the advanced mode pane:

So in this example the button does not display if the Status column is Completed:

The full JSON code for that is:
{
"$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
"elmType": "button",
"txtContent": "Send Email",
"customRowAction": {
"action": "executeFlow",
"actionParams": "='{\"id\":\"021ec4e0-8f85-46b3-a557-a4456960a4eb\", \"headerText\":\"Email re ' + [$Title] + '\",\"runFlowButtonText\":\"Go\"}'"
},
"style": {
"background-color": "#b0c4de",
"border-radius": "10px",
"visibility": "=if([$Status] == 'Completed','hidden','visible')"
}
}
Rob
Los Gallardos
Principal Consultant, SharePoint, Forms and Power Platform, WSP Global (and classic 1967 Morris Traveller driver)