Not enough info.

Describe, in what way you are creating Mensural Notation.

Are you - e. g. - using Robert Piechaud’s Medieval plug-in ?

What difficulties are you experiencing ?

In short:

What are you trying to achieve?

How are you trying to do it?

What happens when you do that?

What doesn't happen that you expect to happen?

Also, it might help if you attach a graphic showing the layout you need.

The better we understand, the better we can help.

I use a hard space between the lyric syllables. and place a custom elision ligature shape expression there.

Sorry, been away from my desk.

I take the point of "not enough info" from Peter Thomsen

I am editing a 1556 edition of white note mensural notation into modern notation.

The difficulty I have is in creating a ligature sign.  I have tried using the smart shape horizontal line with two verticals but this doesn't really work,

How can I go about creating the ligature sign?

Windows 10 Finale 26 (26.3.1.520)

I do this all the time, using the Smart Shape horizontal bracket with the endpoints facing down.  I keep my all my ligature brackets horizontal and at the same height, as much as possible.  If you attach the endpoints to beats, the brackets will stretch to accommodate resizing of the measure (you apparently can't have this shape attach to notes or noteheads).  I also use the "Snap to Beats" function when entering the bracket.  When the bracket is entered with the snapped values, I then run a Finalescript that uses the JW Change plugin both to align all the brackets to the same vertical plane, and to nudge the right endpoint to where I want it, slightly different for whether the stems of the enclosed notes are up or down.

What's nice about entering the brackets this way, is that you can copy any bracket that encloses the same rhythmic values anwhere else in the score and the alignment and attachment points will accompany it.

I am sorry you have been experiencing problems.  What doesn't work for you?

WIndows 10, Finale 26.3