It is possible to write a variable value directly from a flow or dashboard, without entering the value in "write panel" node. Here is a solution using standard nodes : A slider is used to set the value (but can be done by any node) then the “template” node formats the payload, and the change node set the topic.
Details :
The template node : 
The variable address and variable type must be filled here.
For the type and address, the easiest way it to try first with a write panel and a debug node, and see the values.
The Payload value from input will be inserted in the template. (the value to write)
The next node is a “change” node to set the topic to “write” 
The result :
When the slider is moved, the new value is written to g_constDouble