This idea is under review and may stay like this for a while. The DeepL API does not support this feature so it's not available anywhere other than their own web interface. This article confirms: https://support.deepl.com/hc/en-us/articles/4407359201938-Selecting-alternative-translations and this part is relevant:
"Alternative translations are not available for the translation of whole documents or supported via the DeepL API/CAT tool access."