Hi, thanks for submitting a question.
The following documentation article describes one pattern for automatically opening your add-in when opening a document in Excel on the web: https://learn.microsoft.com/en-us/office/dev/add-ins/excel/pnp-open-in-excel
If that article doesn't help, how are you deploying your add-in? The following article has a list of the possible deployment methods: https://learn.microsoft.com/en-us/office/dev/add-ins/publish/publish
Cheers,
Alison