Use parentheses and string literal concatenation:

msg = (
         f'Leave Request created successfully. '
         f'Approvers sent the request for approval: {leave_approver_list}'
)

Note, the first literal doesn't need an f, but I include it for consistency/readability.

You will need a line break unless you wrap your string within parentheses. In this case, f will need to be prepended to the second line:

'Leave Request created successfully.'\ 
f'Approvers sent the request for approval: {leave_approver_list}'

Here's a little demo:

In [97]: a = 123

In [98]: 'foo_'\
    ...: f'bar_{a}'
Out[98]: 'foo_bar_123'

I recommend juanpa's answer since it is cleaner, but this is one way to do this.