Here is the step by step process for adding Dynamics 365 Marketing to your dynamics 365 instance.
You can sign up for, install, and use a trial version of Dynamics 365 Marketing for free. Trials last for 30 days.

Few important points to remember :

• You will receive a trial Dynamics 365 instance together with your Marketing trial. You must run the Marketing trial on a trial instance, and can’t install it on a production instance.
• You can’t convert a trial instance into a production instance.
• You can install, at most, one Dynamics 365 Marketing trial per Microsoft 365 tenant.

1. Go to the admin platform of your tenant:  https://admin.powerplatform.microsoft.com/  
   Expand Admin centers and click on Dynamics 365:
   
   
   
2. In Dynamics 365 Admin centre, click on Applications tab  –> Select Dynamics 365 Marketing Application  –> Click on Manage:
   
   
   
3. Otherwise access this url directly :  https://port.crm8.dynamics.com/G/Applications 
   [Replace crm8 with appropriate region]. Follow the steps given on step 2. 
   
   
4. On the next screen you will be asked to provide a portal name, Please note that a portal is needed to host your pages which can be accessed by customers publically. In this installation a portal of type “Events” will be created for us.
   
   
   
   
5. On the next screen, wait a few seconds:
   
   
   
6. Another important step : You must provide a physical address of the organization ; In our case its a trial, you can put it in anything and also accept the data consent.
   
   
   
7. The installation will start, though it says it will take approx 10 mins, I would suggest you leave it for sometime and come back later. 
   
8. Once that is done, You will see something like below:
   
   
   
   
9. And its done! Now to access the app, you can click on “Go to app” from the above screen or simply navigate to “Marketing” App from your dynamics 365 instance navigation as it will start appearing post installation:
   

Hope this helps.

Cheers!