In Sourcetree, you can select the commit for which the message needs to be modified and then choose "Interactive Rebase.." from the Repository menu.
Use "Edit Message" during the rebase to include the JIRA key and click "Ok" to save the change.
Hi @Eugene Pavlovs,
You are able to enter whatever you'd like in the commit message editor in Sourcetree, even Markdown on macOS. There is no automated way to insert a ticket's reference into it from Sourcetree currently.
Right now it's more of a workflow problem. The way we do it on Sourcetree is to manually copy and paste the title as the first line of the commit message. Which then becomes the PR title and then the merge message in Bitbucket Cloud.
Principal Developer, Sourcetree