setting up a value for a variable name in thymeleaf

You can use local variables.

Declare an HTML element with a th:with attribute. For example

<div th:with="someVariable=${someValue}">

The documentation states

When th:with is processed, that [someVariable] variable is created as a
local variable and added to the variables map coming from the context,
so that it is as available for evaluation as any other variables
declared in the context from the beginning, but only within the bounds
of the containing tag.