For now, bitbucket does not support the feature to create pull request from original repo to fork repo (only the opposite way is supported: pull request to merge changes from fork repo to original repo).
And I create an issue Enable to create pull request from original repo to fork repo to suggest this feature, you can follow up.
The workaround for now is merging changes from original repo to fork repo locally and then push to the forked repo. Detail steps as below (such as merge the changes from UAT master
branch to Preprod master
branch):
#In the local Preprod repo
git checkout master
git remote add upstream <URL for the UAT repo>
git pull upstream master
#Commit conflict files if there has
git push origin master
Now in the Preprod repo master
branch, it contains latest changes from UAT master
branch.