Make sure you add the correct Bitmex Interest Base and Quote Symbols in the input settings
For example for www.bitmex.com/app/contract/XBTUSD the inputs are:
Chart ticker: XBTUSDPI8H
Interest Base: XBTBON8H
Interest Quote: USDBON8H
In true TradingView spirit, the author of this script has published it open-source, so traders can understand and verify it. Cheers to the author! You may use it for free, but reuse of this code in a publication is governed by House Rules. You can favorite it to use it on a chart.