HAR (HTTP Archive format) is a JSON-formatted archive file for logging a web browser’s interaction with a site.
Instructions
Google Chrome
Open the Developer Tools from the menu (Menu > More Tools > Developer tools) or by pressing F12 (or Fn-F12) on your keyboard.
Click on the Network tab.
Look for a round button at the top left of the Network tab. Make sure it's red. If it's grey, click it once to start recording.
Turn on "Preserve log".
You can use the clear button (a circle with a diagonal line through it) right before trying to reproduce the issue to remove unnecessary header information.
Reproduce the issue.
Save the capture by right-clicking on the grid and choosing "Save as HAR with Content".
Mozilla Firefox
Start Firefox Developer Tools in Network mode (Top right menu > Developer > Network, or Ctrl-Shift-E / Cmd-Alt-E on macOS).
Turn on "Persist logs" on the right.
Reproduce the issue.
Save the capture by right-clicking on the grid and choosing "Save all as HAR".
Microsoft Edge
Open the Developer Tools from the menu (Menu > More Tools > Developer tools) or by pressing F12 (or Fn-F12) on your keyboard.
Open the Network tab.
Look for a round button at the top left of the Network tab. Make sure it's red. If it's grey, click it once to start recording.
Turn on "Preserve log".
Reproduce the issue.
Save the capture by right-clicking on the grid and choosing "Save as HAR with Content".
Note: When you record a HAR file, all the internet traffic from that browser tab is captured. This includes cookies and sensitive data.
Source: HAR Analyzer