How do I input my BMI into Garmin Connect?

You can import the Body Fat % with a simple CSV file in Garmin Connect Web at https://connect.garmin.com/modern/import-data. 

This is a sample CSV file:

Body
Date,Weight,BMI,Fat
"2022-11-03","71.6","22.6","14.4"

There can be multiple lines of data, but the first 2 header lines appear just once. Use a plain-text editor such as Notepad for creating the file. Do not use any rich-text editors like Word or Wordpad. And avoid Excel.