Hi K2A,

Yes It's possible to use Javascript in k2 designer please follow below steps

1) Add Data Label to the design canvas.

2) On R.H.S. for Properties panel under General do the following changes

• Check Literal option
• For expression click on browse button
  In expression popup Click on Add and Paste your Javascript.

While doing this Please remember to add your script inside <script></script> tag

Example:

<script>alert('Hello !!');</script>