No, you don't go to the court and ask the judge to sign the motion on the spot. Here's what the Texas statute says:
(a) In a nonjury trial or at a hearing, on the application of a party, the amicus attorney, or the attorney ad litem for the child, the court shall interview in chambers a child 12 years of age or older and may interview in chambers a child under 12 years of age to determine the child's wishes as to conservatorship or as to the person who shall have the exclusive right to determine the child's primary residence. The court may also interview a child in chambers on the court's own motion for a purpose specified by this subsection.
(b) In a nonjury trial or at a hearing, on the application of a party, the amicus attorney, or the attorney ad litem for the child or on the court's own motion, the court may interview the child in chambers to determine the child's wishes as to possession, access, or any other issue in the suit affecting the parent-child relationship.
(c) Interviewing a child does not diminish the discretion of the court in determining the best interests of the child.
(d) In a jury trial, the court may not interview the child in chambers regarding an issue on which a party is entitled to a jury verdict.
(e) In any trial or hearing, the court may permit the attorney for a party, the amicus attorney, the guardian ad litem for the child, or the attorney ad litem for the child to be present at the interview.
(f) On the motion of a party, the amicus attorney, or the attorney ad litem for the child, or on the court's own motion, the court shall cause a record of the interview to be made when the child is 12 years of age or older. A record of the interview shall be part of the record in the case.
Tex. Fam. Code Ann. § 153.009. Note that if the child is at least age 12 then in at least some circumstances the court must interview the child in chambers if an application is made by any of the persons specified in the statute. Note, too, that it appears the application/motion may be made orally at the hearing or trial. If you submit the request in a written motion before the hearing or trial, you'd submit that like any other motion you file with the court. What is the best way to pursue this in any given case, though, is something that is going to be a matter of local practice — the judges of each court may have differing practices on this. If you are unsure what to do then I suggest you meet with a family law attorney for advice on it.