Hi Johnson,
Content assist only works in Marketing Email, you could take solution below to show contacts' email address in marketing page.
1. It is required that "Prefill marketing form" field of Contact is set to "Allow".
2. Create a new personalized page record, add emailaddress1 into Fields subgrid.

3. Copy the automatically generated code from step 1 to marketing page.
If there is an embedded form-loader script in source code of marketing page, then we only need to copy window.msdyncrm_personalizedpage and its callback function.
In my page, I append the code after end tag of body.

Add a span element to display emailaddress1 value.

var emailaddress1 = data.Data["emailaddress1"];
document.getElementById("email").innerHTML = emailaddress1;
Result:

Please read article below for more details about how to use "Personalized Page".
https://docs.microsoft.com/en-us/dynamics365/marketing/personalized-page-content
Regards,
Clofly