Hello @geirmat,
To redirect user to specific Form Set on button click, you can add this code to button's Click settings:
//replace 'c46596f4-3325-43d5-86f9-77fc72d52ad4' with your FormSet Id
localStorage.setItem('FormSetId', 'c46596f4-3325-43d5-86f9-77fc72d52ad4');
location.reload();
And add this code to Custom Routing:
var formSetId = localStorage.getItem('FormSetId');
if(formSetId){
localStorage.removeItem('FormSetId');
return formSetId;
}