Hi @Anonymous ,
The red part is decided by whether you want to create a new record or edit an existing record.
If you want to create new record, you need to use this formula:
Patch(tablename,Defaults(tablename),{....})
If you want to edit an existing record, you need to use this formula;
Patch(tablename,the item that you want to edit,{....}) // you could use many formulas to represent the item that you want to edit, for example: LookUp,First(Filter()),gallery.selected.
And the formula that you use "ThisItem" is ok too.
Here's a doc about Patch function for your reference: (I think maybe you do not kown a lot about this function)
https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/functions/function-patch
So could you tell me your error message?
Best regards,