Usually buttons set variables with Buttons with hints and Fork which have special variable field. By default there’s no way to set a variable with Button, however variables can be set with Assign a variable component.
1. Add Buttons.
2. Add screens where these buttons will forward to.
3. Add screens that have Assign a variable components.
4. Set Variable Name to the variable which will take the pressed button.
5. Set Variable value to the text that corresponds to the pressed button.
6. Remember to save the project.
Pressed button value is set the variable.