You could try the following:
- Give your input widget a (unique) class in the appearance tab
- Create a button that calls a nanoflow
- In the nanoflow call a javascript action with a class parameter that you fill with the unique class name for your input widget
- In the javascript action, use code like this:
document.getElementsByClassName(YOUR_CLASS_PARAMETER_HERE)[0].getElementsByTagName('input')[0].focus();
Alternatively, use a htmlSnippet widget with html like
<button onclick="document.getElementsByClassName('YOUR_CLASS_HERE')[0].getElementsByTagName('input')[0].focus()">BUTTON LABEL HERE</button>