Hi @Amer,
I expect you likely want to be asking about this in the more general ArduPilot forums - the Blue Robotics forum is specifically for discussing marine robotics, and we don’t use Mission Planner so can’t provide support for it. They also will have more experience with Lua scripting, and likely with Pymavlink as well.
Note that this can pad a name out to be 10 bytes long, but it will not reduce the length of values that are over 10 bytes, and 'Battery1 voltage' is 16 characters long.
If it’s relevant, I have an example from several years ago for sending a NAMED_VALUE_FLOAT message with Pymavlink, which I managed to receive correctly in a custom build of QGroundControl. Nowadays that would be substantially easier using Cockpit and it’s Very Generic Indicator widget, but if you’re not running BlueOS on an onboard computer on your vehicle then Cockpit is quite challenging to set up at the moment.