The parameters can be set only on second (and subsequent) builds).
Steps to reproduce :
Expected behavior : the link should be "Start a build with parameters", and should let the user give the parameters
Workaround : start a first build, and interrupt it after the git checkout. Then the link correctly becomes "Start a build with parameters"
Are you still facing this issue Mossroy?
Yes I am.
Same behavior with version 1.4.4 of the plugin, and gitlab 12.6.2.
I might try with version 1.4.6 of the plugin if you like, even if the changelog does not mention anything related.