Hello Loux xavier,
You will need to slot the button as a horizontal box slot, from there you can pull off the return value of the “set as horizontal box slot” node and use the “Set Size” node to set the button to fill. I have provided an example below. I hope this helps.
Example:
I got a reference to the button and slotted it as a horizontal box slot. From there I set the size rule to fill.
As a note: You will need to split the pin for the node to look like the example above.
Make it a great day
