Here link video streaming address Can someone tell me the interface?

Mission planner

where is the mission planner installed ? Is it on desktop or on your radio telemetry transmitter.
What is your radio telemetry transmitter brand ?

here link

it should work without any effort.

I need interface information, but the manufacturer doesnโ€™t tell me, and there is no API interface for me.

I suggest you use UART to first check whether Mission planner connects to your drone . This should work straight out of the box. Make sure Herelink and your drone flight controller are in the same network
Unfortunately my Herelink is sitting somewhere else and i will have it beside me after Monday

There is no problem with my hardware connection and control. Now I want to call their API for my own app development, but I canโ€™t get effective information

I know only Ardupilot and Px4 to be compatible with Herelink. But your third party app may be the issue. How do you know that it is providing the required Mavlink messages ?
Can you test your third party app throโ€™ a companion computer like Raspberry Pi by installing Mavproxy on Pi and using mavproxy messages

target sysid is 42
target compid is 100

i also question the mavlink library you are using, as compid is not a valid field in msg_command_long. this means that the crc is likerly incorrect as well

sysid = 42 compid = 100 is not working

image at the sametime we use msg_command_long to set servor ,it is working.

Hi
Please , were you able to get the HDMI port 2 working .

thank you