How do I know when someone assigns an issue?

[Ana Clara do Prado da Silva Coelho]

Hello,

I'm trying to find out when someone has changed the issue from unassigned to assigned. I attempted to modify the 'first_answer' field, but it didn't work. Then, I used the endpoint:

"{company}/rest/api/{version}/issue/{key}/changelog"

However, I only received the correct results in some cases; most of them returned NULL/NONE. Does anyone know how I can configure the SLA (first answer) or which endpoint I can use to obtain this information? Please.