I am working in with a Trust Game Experiment and someone suggest me that I can use an alternative measure. He give me a paper of Altmann et al. (2008) "Do the Reciprocal Trust Less?" and his work said that

"We measure trust by the amount that a subject sends as a first mover. Our measure of reciprocity (also denoted “r”) is derived as follows: for each subject, we used the decisions as a second mover and ran a simple OLS-regression of the amounts sent back on the (hypothetical) amounts sent by the opponent, forcing the slope through the origin. The slope coefficient gives us a measure of a subject’s willingness to reward kind actions of an opponent by own kind behavior, i.e., positive reciprocity. If a receiver, for example, always matches his final payoff with that of the sender, his reciprocity coefficient is r = 2."

How can I run this OLS-regression in Stata and which variables can I use?

Thanks for your help