How to call a service from XML launch file?

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For posterity, I found a solution using the executable tag. Let's say you wanted to spawn a second turtle in turtlesim, the below will accomplish this:

<executable
    cmd="/spawn turtlesim/srv/Spawn '{x: 1.0, y: 1.0, theta: 0, name: 'custom_turtle'}'"
    cwd="/home"
    launch-prefix="ros2 service call"
    output="screen"
    >
</executable>

I don't know what 'cwd' is used for but it should remain as "/home" or else this particular command won't work. You can also combine the launch-prefix and cmd if so desired.