Use a template node and feed that into the service call node. Note that the output type should be json and that you don’t need msg.xyz for the template, just xyz works.
I believe what you would want would be
{ "data": { "entity_id" : "input_number.slider1", "value" : {{payload}} } }
EDIT: to further add, you can use the function node as well and write in javascript. The template node is probably the best option unless you need to do transformations on the data.