How do I spawn a large platform of lava up at build limit?

I'm looking to use a command block to spawn in a large platform of lava up at the build limit so it will flow down. I'm having trouble with the commands, though, and the fact that the lava doesn't flow. Is what I'm trying to do possible or not?

When liquids are manually spawned into the world, they can be still or flowing. A flowing liquid block will attempt to flow into adjacent blocks as per the usual rules. A still liquid block will not do this. Note that the flowing-vs-still distinction is completely separate from whether or not it's a source block, and has nothing to do with depth, the flowing animation, etc.

Still liquids are called "water" and "lava", while flowing liquids are called "flowing_water" and "flowing_lava". You want to spawn "flowing_lava" instead of "lava".

The reason Minecraft has two different kinds of block is because most of the time, liquids don't need to flow right after they spawn in the world (e.g. as part of an ocean or lake). It would be inefficient to trigger large numbers of block updates whenever new ocean terrain were generated. So Minecraft spawns still water which does not attempt to flow anywhere. But sometimes, liquids spawn in places where they do need to calculate real flow (e.g. waterfalls), and these are spawned as flowing liquid blocks. All flowing blocks transform into still blocks once they've done the flow update, and still blocks transform into flowing blocks when a block update happens in an adjacent block.