TC Source indexing fails with message 'No source information found in pdb file'

Hi,
I'm testing TC and trying to implement IC for my company that using Mercurial VCS, especially the symbol server and source
indexing capabilities. My company uses Mercurial server from an online third party.

I'm be able to compile my VS .Net solution and store symbols with TC. The pdb artifacts are well publish.
But I can view in the build log that source indexing step is not correct. I have This log message for all my pdb files :
"Indexing sources appeared in file {pdbfilepath}".
"No source information found in pdb file {pdbfilepath}".

My localhost TC Server test configuration :
- TC 9.0.4 with Symbol Server Plugin 1.0.6
- OS : Windows 10

When debugging in VS (after configure it), the pdb is loaded correctly from server symbol, but when VS walkthroughs a cs
source, the file came from the agent working directory ({TeamCityFolder}\buildAgent\work\{randomnumber}\...), not
from my mercurial SC server. I supposes that it because source indexing step failed ?

Is this scenario supported by TC and mercurial source server ? If yes, how can I process for that please ?

In the past I'm used TFS and publishes symbols and source indexing capabilities. When I debug it, in the
output window I can view a command line like that :
srcsrv tf.exe ...

There is a way to configure srcsrv for TC indexing source from Mercurial capabilities ?

Thanks!