Hi,
Thank you for your query.
I will recommend following approaches depending on the scenario:
1 - If you only need to display this hyperlink and do not need to store as data, you can create an HTML Web Resource and display it in the form. Here are steps to follow:
- Go to Advanced Settings > Customization -> Solutions.
- Create a new Solution or you can use Existing one. You can also do this in Default solution.
- Click on Web Resources and create a new Web Resource with following properties:
- Name: <type an appropriate name>
- Display Name: <type description>
- Type: Webpage (Html)

- Then click on Text Editor, here you can design your page as per your needs. Or you can simply create a Hyperlink using GUI features (choose External Resource). Here is a sample HTML:

Google Docs
Please note, target="_blank" in the following line will open the link in the new tab.
<a href="">https://google.com" target="_blank">Google Docs</a>
- Save the Web Resource and click on Publish.
- Now, Click on Entities then Add Existing and choose Opportunity entity, and under sub-components choose the form you use.
- Open the form and add the Web Resource control from the ribbon (if you do not see Web Resource option, maximize the form designer window), type a name for the control and choose the Web Resource created. In the Formatting tab, you can choose the appropriate number of rows, for this requirement 1 row should be fine.


- Save and Publish the form.
2 - If you need to store this link with Opportunity data (for reporting or other purposes):
- Create a Field in opportunity entity with following properties:

- Add the field to the form, and click on Business Rules.
- Add a new Business Rule, change scope to Entity, this will make sure Business Rule is executed on all forms and all sources (Web, API).
- Add the following condition:

- Then on the True (checked) path of the condition add Set Field Value Action:

Here is the final look of the Business Rule:

- Save, Validate and Activate the Business Rule.
- Save and Publish the form.

The first link is using Web Resource and other is using field and a Business Rule.
Please note, you can also create a rea-time workflow to update the existing records with link value (for the second approach). This workflow can be executed on demand.