Description
is a plain-text field and as such it won't parse HTML. Generally speaking, the solution is to add a custom field of correct type to your object and populate this field. A field type that allows HTML and/or other markup is Text Area (Rich):
Text Area (Rich)
[...] users can format the field content and add
images and hyperlinks. [...] The maximum field size is 131,072
characters, inclusive of all the formatting and HTML tags.
Unfortunately Activities (Task is a type of Activity) do not allow custom fields of Text Area (Rich) type. Another field type that is available for custom fields on Activities is URL
:
Allows users to enter up to 255 characters of any valid website
address. Only the first 50 characters are displayed on the record
detail pages. When a user clicks the field in Salesforce Classic, the
URL opens in a separate browser window. In Lightning Experience,
internal URLs open in the same window and external URLs open in a
separate browser window. In Salesforce console apps, the URL opens in
a new workspace tab. In Lightning console apps, internal URLs open in
a new workspace tab and external URLs open in a separate browser
window.
Instead of HTML markup in one field, you could have plain text in the Description field with the instruction to click the link contained in your custom URL-typed field.