How to incorporate Javascript into method or custom keyword

Well, one way is to extract your JavaScript out of your sin file and drop it right between the two sets of triple-single-quotes (much as I hinted at above)

'''
line 1;
line 2;
etc;
return mySin;
'''

That assumes there is a JavaScript var called mySin.

Now you wrap the triple-quoted code block in a call to executeJavaScript, something like…

String mySin = WebUI.executeJavaScript(
'''
line 1;
line 2;
etc;
return mySin;
''',
null)

Notice I’m not using WebElement - I don’t think that’s what you want… I think your SIN is a string of digits, right?

Now you can do…

WebUI.setText(findTestObject(...), mySin)

or whatever.