I have a requirement where I have to populate multi select control from the data in termset, if the value isnt there in the the available data then user would be able to add the value to term set using textbox control
Could you please share a bit more about your scenario?
Could you please share more details about the data source you used within your app?
Further, could you please share a bit more about the Term Set you mentioned? Is it the term set within the Managed Metadata type column in SP list?
If the term set that you mentioned is a Managed Metadata type column in your SP list, and you want to add a new value to pre-defined term set using Text Input box, I afraid that there is no way to achieve your needs in PowerApps currently.
Currently, we could only choose value from the pre-defined term set within PowerApps app. If you would like this feature to be added, please submit an idea to PowerApps Ideas Forum: