Approach to set some values programmatically

[clivezhg]

I'm trying to make a Wordpress plugin support YoastSEO, and I would like to set some values programmatically, but I'm not able to do it.
E.g., to change "Meta description" programmatically, according to my investigation, I suppose the following steps can work:

1. $("#yoast_wpseo_metadesc").val("Some Value");
2. window.YoastSEO.app.reloadSnippetText();
   or 2. window.YoastSEO.app.refresh();

Hower, the above doesn't work. It looks that the 'window.YoastSEO.app' doesn't have the 'snippetPreview' member might be the cause.
Could anyone help? Thanks a lot in advance!