I recommend doing that from your online account. THunderbird is not designed for that

I recommend doing that from your online account. THunderbird is not designed for that

Definitely a good choice, if the vendor of the online account supplies that capability.

If not, Thunderbird can generate replies by using Thunderbird filters and template messages (see screen shot), saved from the compose window with Save > Template.

https://addons.thunderbird.net/en-US/thunderbird/addon/filtaquilla/ might also offer advanced filtering that could help.

I do not get an option appearing saying reply with template. Do I need some sort of add-on? See Screenshot

I do not get an option appearing saying reply with template. Do I need some sort of add-on? See Screenshot

Reply with template is not shown if you do not have a template folder for that account.