Hello @Martin Kotrc
Thank you for reaching out. Currently there isn't way to set a default custom message when inviting guest users via AAD. You would need to add a specific custom message every time you invite guest user via Portal. However to use same message again and again you can use Azure AD PowerShell script to achieve so. You can review following screenshot to invite user with custom message from Azure Portal and a reference QnA post with working PowerShell script to invite user with custom message.
Azure Portal:
Azure AD PowerShell Script available on following QnA post: Invite Guest users with custom invitation message in AAD using a powershell
I hope this helps and answers your query. Please "Accept the answer" if the information helped you. This will help us and others in the community as well.