How to add a tab Developer inside PowerBI Desktop?

Hi toplisek 
Thank you for reaching out microsoft faric community forum.

Power BI Desktop doesn’t have a "Developer" tab like Excel does. But if you're looking for developer-related features, there are a few tools within Power BI that might be what you need:

• Advanced Editor – You can find this under Home > Transform data > Advanced Editor. It lets you view and edit the M code behind your queries.

• DAX formulas – In the Data or Model view, you can create measures and calculated columns using DAX.

• Performance Analyzer – This is available under the View tab and is great for checking how your visuals are performing.

• Custom Visual Development – If you're looking to build your own visuals, you can use the Power BI Developer Tools outside of the app. Here's a link to get started:
  Power BI Developer documentation
  
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