You can simply draw the horizontal line on the indicators also from the drawing tools or using keyboard shortcut Option(⌥) + H in Mac or Alt + h in Windows.
That would be a temporary line which a trader will be required to draw everytime for every stock @Naman
Trading View provides an option to define the upper and lower band for RSI which is an ideal way to set by a trader so that the preference defined by him/her becomes permanent.
The RSI we have on dhan charting platform is inbuilt in the tradingview library, we do not have much control over it. I think you are talking about BB provided in RSI in updated library. We have raised the request with TV, and hope we get this update soon.