How can I generate a URL to a Post Event out of the Post Entry Id?...

The url is:
<appian server>/suite/tempo/entry/<Post Entry Id>

You can create expression as shown in attachment. You will get Tempo Id from 'Post Event to Feed' output tab which will be used in the given expression..

It is not necessary to put the name of the appian server at the beginning ?

Yes... prefix 'suite' to SALES_linkToTempoPost expression rule
See attachment.

no need to put machine name . It takes context automatically

Great! Thank you.