If you want it to grow more trunk (ie get thicker), you will need to repot it this summer (Probabably mid June-July4thish in your area) into a larger pot.
Pick a shallow/wide pot to promote radial roots instead of deep straight down roots. Dont go rediculously large, just larger, something like this:
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The 11" or 14" length pot. Not sure how big that tree actually is but you could start with the 11 inch then repot into the 14 when it grows bigger.
Looks like its about a 1 inch trunk (?) so Id start with the 11 inch pot and see how it fits in that.
And let it grow as Choppy said. Feed it, BRT are hungry plants so feed them well during the growing season and it will grow.
Also provide it with supplimental light during the winter which will also help it to grow.
Here are my BRT in their winter quarters:
