How can I send commands to the virtual machine through the API interface?

Actually, I do not use Postman but you can check it from the curl command for example:

Bash:
curl -s -k -X $'POST' -H $'Csrfpreventiontoken: ${CSRF}' -b $'PVEAuthCookie=${Cookie}' $'https://10.10.10.24:8006/api2/json/nodes/pve/qemu/101/agent/exec?command=df+-h'
'

You can also intercept the request on Postman using --proxy flag e.g:

Bash:
curl -s -k -X $'POST' -H $'Csrfpreventiontoken: ${CSRF}' -b $'PVEAuthCookie=${Cookie}' $'https://10.10.10.24:8006/api2/json/nodes/pve/qemu/101/agent/exec?command=df+-h' --proxy 127.0.0.1:8080
'