The ChimeraX AlphaFold interface does not allow specifying your own structure template.  But the ColabFold web server does allow this

using "template mode: custom" on that web page and follow instructions they list on that page for using custom templates

  I have tried custom templates with AlphaFold in the past and it has never changed the predicted structure.  AlphaFold only uses the template as a first guess at the structure then it does multiple rounds (by default 3 recycles) of creating new models.  If AlphaFold is pretty confident about a different conformation than the template it simply replaces the starting structure with the conformation it prefers.  Possibly if AlphaFold has low confidence in a prediction a template could bias it to get a different conformation but I've never seen that happen.  This is a basic limitation of AlphaFold that it is not designed to optimize an existing structure.  The input that dominates the calculation is the sequence and the deep sequence alignment that it computes, not template structures. Possibly AlphaFold could be prodded to predict different conformations by changing the deep sequence alignment to include just the sequences known to fold in the desired conformation and excluding sequences known to fold in a different conformation.

Tom