Yes, if you go to the website, and start typing, it defaults to the Detect language option.
But what I’m talking about is customizing the url, so you don’t need to go to the site, to start your search.

If you go to https://translate.google.com/?sl=auto&tl=en&text=%C2%BFHola, cómo estás? it will automatically detect that the string is in Spanish, because of the auto in the url.

That’s what I’m looking for with DeepL.

EDIT: Damn, seems like the parameter used to be xx, but that it’s been broken for some time now, if this exchange in January on stackoverflow.com is to be believed. Or the parameter was . and/or auto, according to this Reddit thread back in November, where they also talk about it being broken.