Hi there @v-kaustubhp, welcome to the Community!
Which version of Jira are you using? There’s a Jira Software Cloud app and a Jira Software Server app so want to make sure we’re talking about the same Jira app here.
That said, both of those apps appear to have Add Watcher to Issue actions which should allow a Zap to add a watcher to an existing issue (ticket?) in Jira. But I’m not seeing any options to add a label to an existing issue, so you may need to contact our Support team to request that a feature request for that to be created on your behalf with the relevant Jira app.
In the meantime, it looks like you might be able to use a API Request (Beta) action event to edit the issue and add a label to it. It’s a bit more advanced so I can’t give you exact step by step instructions on how to do it here, but we have a more general guide that you might find useful: Create an API request in your Zaps (Beta)
And depending on which app version you’re using you’d need to reference one of the following API guides:
Hope that helps to point you in the right direction, please do let us know how you get on with this!