Anyone help me with API integration

can anyone help me integrate to custom domain
here’s the documentation link -

can anyone guide me for this please

Sorry but I can’t find custom domain integration only base url provided

custom domain is not the issue I have my domain, I want to integrate Reemo API to my website,
I have purchased there API plan and I received API key and a security key but I’m not able to understand how do I integrate or connect to my website, Right now I do not have a website designed I just have a domain.
can you guide me how do I integrate using

here’s the documentation link they have provided - the process of integration but I’m not able to understand.

Hope you understand what I’m trying to say
And sorry for not giving a brief previously