Hi Jolas365,
1. Either embedded marketing form or captured form can be used for external CMS, while the first solution is more recommended, because it supports custom subscription center functionality.
You can refer to table in following documentation for comparison between those two types forms.
Embed forms on external websites (Dynamics 365 Marketing) | Microsoft Docs
2. As I answered in question 1, it is possible and there is two ways, however, due to you subscription page is required to be hosted in an external website, thus the embedded form would be the only option.
3 and 4:
Prerequisite:
You need to have access to domain which is used to link to page in Umbraco CMS.
If the page is hosted on public domain of the CMS provider(e.g:jolas365.umbracocms.com), then the whole process couldn't be achieved.
In other word, Umbraco should give you ability to bind you own domain to its page.(jolas365.com)
So please make sure to meet this most important requirement.
step 1: Authenticate your domain in Marketing application.
Domain authentication (Dynamics 365 Marketing) | Microsoft Docs
step 2: Create a subscription center type marketing form, add "Do not email" field and subscription lists to the form, after going live it, create a new form page record in Form hosting tab.
Set up a subscription center (Dynamics 365 Marketing) | Microsoft Docs
step 3: In the new form page record, copy the automatically generated script.
step 4: Paste the script to Umbraco CMS page.
step 5: Create a new content settings record, set Subscription center field to the URL of "customer portal" page.
step 6: Before going live a new customer journey, set Content settings field(in General tab) to the new custom content settings.
step 7(final): Contacts will be navigated to your own subscription center page after they clicking the subscription link in email, then they can manage their subscription by checking/unchecking subscription lists checkboxes.
Note: Please run a simple customer journey to test subscription center instead of "test send".(But "send now" is ok.)
In a word, contacts unsubscribe by checking the "Do not email" field of the subscription center form, and since then marketing email will no longer be delivered to them.
Regards,
Clofly