I added the UML Sequence shape library, following this document: https://help.lucid.co/hc/en-us/articles/16262874090900-Create-a-sequence-diagram-with-UML-markup-in-Lucidchart
Step 2 of this document shows the library and it has a text link to Use Markup as per this screen shot:

In my environment this link is missing:

Please advise