For message page url in email you can set “Front End PM Page” in Dashboard > Front End PM PRO > Settings > General.

To auto fill your selected users, you can use https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/only-admin/ ( users do not have to be admin, you can use normal users also in that list).

If you want to pre-populate (without click), you can use https://www.shamimsplugins.com/docs/front-end-pm/customization/user-name-pre-populate-new-message/ , for multiple users you can pass their id’s separated by comma.

You can even use https://www.shamimsplugins.com/docs/front-end-pm/shortcode/fep_shortcode_new_message_form/ (here you can easily change value by JS).

If you want to select onclick (what you are showing in your screencast) you can use selector.tokenInput("add", {id: x, name: y}); where “id” is user id and “name” is user display name. If you want to select one recipient only, you can call selector.tokenInput("clear"); before adding.

Let me know.