link to a PowerPoint on a iMessage

Hello Flying_alpaca_,

Thank you for reaching out to Apple Support Communities.

These steps will guide you on sharing via iMessage:

"Send a message on Mac

Messages on Mac don’t have to be boring. After you set up your Mac, you can send messages to one person, a group of people, or a business and include text, photos, animated effects, and more. You can express yourself in all sorts of ways:

• Tapbacks: See Use Tapbacks.
• Photos and videos: See Send photos and videos.
• Photos, scans, and sketches (from an iPhone or iPad): See Insert files from iPhone or iPad.
• Stickers and images: See Send stickers, Use #images, and Create your Memoji.
• Audio messages: See Send an audio message.
• Message effects: See Use message effects.

1. In the Messages app  on your Mac, click the Compose button  to start a new message (or use the Touch Bar).
2. In the To field, type the name, email address, or phone number of the person you want to send a message to. As you type, Messages suggests matching addresses from your Contacts app  or from people you’ve previously sent messages to.

You can also click the Add button  to the right of the To field. Click a contact in the list, then click the email address or phone number.

Note: If you’re restricted to sending and receiving messages with only certain people, an hourglass icon appears next to those people you can’t send messages to.

3.Enter your message in the field at the bottom of the window. You can use typing suggestions, if available.

Siri: Say something like: “Message Mom that I’ll be late.” Learn how to use Siri.

4.Press Return on your keyboard or click the Send button to send the message."

Additional details can be found here: Send a message on Mac

If the issue persists, can you provide a few more details? What happens when you attempt to share via iMessage?

Cheers!