how to mark lost mode my iphone?

Hi there, otgoo114! Thanks for reaching out to Apple Support Communities. 

After reviewing your post, it sounds like you are looking for some help with putting a missing iPhone in Lost Mode. We definitely understand that can be a concern and we'd love to help. Please review the following articles, which may help with your lost iPhone:

• Use Lost Mode in Find My iPhone on iCloud.com
• If your iPhone, iPad, or iPod touch is lost or stolen
• Use the Find My app to locate a missing device or item

Please let us know if you have follow-up questions about this issue, or if the issue remains unresolved. Have a great day!