By default, only admin can add a Vanity Url on the content. Since 1.7.6, Publisher can do that if allowed by the admins in the configuration.
You should ask your administrator to add vanity url you want. or see, if you have RStudio Connect 1.7.6 or later, with them if they are ok to activate the feature where publishers can customize the URL themselves.
See
https://docs.rstudio.com/connect/user/settings-panel.html#vanity-url
Note: To enable publisher vanity URL permissions, you must be running RStudio Connect 1.7.6 or later. Instructions for the PublishersCanManageVanities setting can be found in the Authorization configuration section of the RStudio Connect Admin Guide.
Hope it helps