Hi,
I've never had to do that yet but I would maybe try something like this:
Have an text input control on the screen and where you can type the Keyword. Then try setting the "Items" property in the gallery to something like this: If(IsBlank(TextInput1.Text),Office365Users.SearchUser(),Filter(Office365Users.SearchUser(),TextInput1.Text = JobTitle))
I actually just tried it and it works! You could also use a similar formula with the same idea if you just wanted a simple combo box. I assumed you wanted a gallery.
Also, instead of putting in the "=" you could have "Contains" which would make the search a little better. Or you could try having a combobox with all the job titles available.
Let me know if you have any questions,
Ben