Button to add attachment based on text in subject

Hello,

I need to create a button which will attach a specific item depending on the text in the subject.

For example the subject could be "Invoice for services (123456)"

I have a macro which creates a new message and attaches the item, but a lot of the time I need it on a reply or forward.
So I just need a button (in the Quick Access Toolbar) which will search the subject of the currently open email which I'm writing and attach invoice 12345.

The attachments will always be saved in the same folder.

I know VBA but for Excel (I am very new to Outlook VBA)

Many thanks,
Matei