Side Loading

link - learn.microsoft.com/en-us/office/dev/add-ins/testing/test-debug-office-add-ins

You can side load your add-ins for quick testing without having to put them into an add-in catalog.
Side loading can be done on Office 365 or Office Online
When you side load the manifest file is it stored in the browsers local storage.

Office 365

Login to your 365 account
Open Excel
Select Insert > Add-ins
Select MyOrganisation
Select Upload My Addin
Browse to the manifest file

Office Online

Open Office Online
Under Get Started, choose Excel
Select Insert > Add-ins
Select the MyAddins tab
Select Manage my Add-ins
Select Upload My Addin
Browse to the manifest file

Outlook Add-ins

add-ins can only be side loaded when you have an outlook.com email address
A gmail email account will not display any side loaded add-ins ?

cd C:\temp\vscode\Outlook-JavaScript\ 
npm run dev-server 

You will/may see the "Security Warning" dialog box appear.
Press Yes to install the certificate.
Open a web browser and login to outlook.office.com
Select an email in your inbox.
In the preview pane on the right hand side, click on the three dots and at the bottom select "Get Add-ins".
Choose My Add-ins on the left hand side.
Scroll to the bottom to see Custom Add-ins
Click on "Add a custom add-in" and select Add from File
Browse to the manifest file:

C:\temp\vscode\outlook-javascript\outlook-javascript\manifest.xml 

You will see a "Warning" dialog box appear
Press Install
Scroll to the bottom of the "Add-ins for Outlook" dialog box to see the add-in appear under Custom Add-ins.
SS
Close the window
Select an email in your inbox.
In the preview pane on the right hand side, click on the three dots and select "outlook-vscode".