so, if you are using METAL, I guess you need to perform meta-analysis of association studies.
Most software performing association studies will provide as a result not only the p-value, but also a beta-estimator, or an estimate of the effect of the risk allele. This is what you need to input as beta-coefficient.
From the METAL wiki:
"The EFFECT column can have positive and negative values (beta values from regression, for example), or simply directions of effect relative to the reference allele, listed as “+” and “-“. An EFFECT of “+” (or any positive number) with respect to the reference allele A (or effect allele A), for example, represents a case where increasing number of copies of allele A are correlated with increasing trait values. For discrete traits, it is common to report odds ratios, which are always positive. In this case, to calculate the direction of effect, one should look at the log of the odds ratio. METAL can compute the odds ratio for you if you specify EFFECT log(ODDS_RATIO_COLUMN)"