Why can't set custom headers?

[lemonl2]

Some websites use headers, for instances

Accept: application/json, text/plain, /
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9,en;q=0.8
Connection: keep-alive
Host: 192.168.21.143
Referer: http://192.168.21.143/team/group/create?edit=0&id=77
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/74.0.3729.131 Safari/537.36
x-auth: fe7902d9-daba-46d2-8090-58e76c036cf1

they use 'x-auth' to authorize user