In my 2020 tax return, I'm able to request missing stimulus payments for 2019, but where to request my missing 2019 tax refund?

I'm still waiting for my 2019 tax refund. Can it be addressed somewhere in my 2020 tax return? If not, and if it's still in processing,  is there any deadline for my 2019's tax refund?   What should I do to avoid it being totally forgotten?