For information on linking git commits to Jira, see Linking git commits to Jira issues.
Create a branch and associate it to a Jira issue
Associate branches to a Jira issue by inserting the Jira issue key to the branch name when creating them.
Select the master branch dropdown.
Enter a new branch name on the search box. Mention a Jira issue key in the name to associate it to the said Jira issue.
Click the result to create the new branch.
Create a pull request and associate it to a Jira issue
Set base and compare as required. (branch to master)
Enter a new pull request title on the provided box. Mention a Jira issue key in the title to associate it to the said Jira issue.
Also, mentioning a Jira issue key on the description will associate this pull request to the said Jira issue.