I do it all the time using "insert quick response" built into Aquamail.
- Start your new email.
- With the cursor in the message area, click on the 3 vertical dots that symbolize "menu"
- scroll down the menu that opens and click on "insert quick response".
- if you've never used it, it may be blank. So, click on "add/edit" and type in the text you want to use. You can also copy /paste it here.
Whenever you want to use this template, click on "insert quick response", click on that text and it will be pasted as a message.
You can have several such texts, not just one. A very useful feature.
Hope this helps.