I have a custom button url to open a Lead conversion page:

/lead/leadconvert.jsp?retURL=%2F{!Lead.Id}&id={!Lead.Id}&nooppti=1

It still displays in lightning but open the convert page in classic! I have tried:

{! URLFOR( $Action.Lead.Convert, Lead.Id) }

Same behaviour, use a page in classic.

{! URLFOR( "/apex/leadconvert", null, [id=Lead.Id] ) }

I got an error message: "Page leadconvert does not exist".

{! URLFOR( "/lead/leadconvert", null, [id=Lead.Id] ) }

I got an error message: "URL no longer exists".

Any idea how to write it to open the page in Lightning. I need this URL to create a Lightning action to do some validation before opening the standard Lightning Component. Thank you in advance for your help. Sylvie