Push feature branch to remote master from Magit

Use C-u C-u M-x magit-push to have Magit ask you which remote and branch you wish to push your current branch to.

With your feature branch currently checked out,
C-u C-u M-x magit-push RET your-remote RET master RET
will push your feature branch to master without having to checkout master first or fall back to git cli.

If you use magit's popup interface, P C-u C-u P will accomplish the same.